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Timeline: The special counsel inquiry into Trump's handling of classified documents

Washington(CNN) The federal criminal investigation into Donald Trump's potential mishandling of classified documents has escalated with additional charges against the former president.

The former president initially faced 37 counts, including 31 counts of willful retention of national defense information, according to the indictment.

Then, the special counsel brought three new charges against him, including one additional count of willful retention of national defense information and two additional obstruction counts.

The investigation -- led by Justice Department special counsel Jack Smith -- revolves around sensitive government papers that Trump held onto after his White House term ended in January 2021. Trump aide Walt Nauta, and a third defendant, Mar-a-Lago maintenance worker Carlos De Oliveira, have also been charged in the case.

Federal authorities have recovered more than 325 classified documents from Trump. He has voluntarily given back some materials, his lawyers turned over additional files after a subpoena, and the FBI found dozens of classified records during a court-approved search of his Mar-a-Lago home last summer.

Trump has denied all wrongdoing and claims the investigation is a politically motivated sham, intended to derail his ongoing campaign to win the Republican 2024 nomination and return to the White House.

Here's a timeline of the important developments in the blockbuster investigation.

May 2021

An official from the National Archives and Records Administration contacts Trump's team after realizing that several important documents weren't handed over before Trump left the White House. In hopes of locating the missing items, NARA lawyer Gary Stern reaches out to someone who served in the White House counsel's office under Trump, who was the point of contact for recordkeeping matters. The missing documents include some of Trump's correspondence with North Korean leader Kim Jong Un, as well as the map of Hurricane Dorian that Trump infamously altered with a sharpie pen.

July 2021

In a taped conversation, Trump acknowledges that he still has a classified Pentagon document about a possible attack against Iran, according to CNN reporting. The recording, which was made at Trump's golf club in New Jersey, indicates that Trump understood that he retained classified material after leaving the White House. The special counsel later obtained this audiotape, a key piece of evidence in his inquiry.

Fall 2021

NARA grows frustrated with the slow pace of document turnover after several months of conversations with the Trump team. Stern reaches out to another Trump attorney to intervene. The archivist asks about several boxes of records that were apparently taken to Mar-a-Lago during Trump's relocation to Florida. NARA still doesn't receive the White House documents they are searching for.

January 18, 2022

After months of discussions with Trump's team, NARA retrieves 15 boxes of Trump White House records from Mar-a-Lago. The boxes contained some materials that were part of "special access programs," known as SAP, which is a classification that includes protocols to significantly limit who would have access to the information. NARA says in a statement that some of the records it received at the end of Trump's administration were "torn up by former President Trump," and that White House officials had to tape them back together. Not all the torn-up documents were reconstructed, NARA says.

February 9, 2022

NARA asks the Justice Department to investigate Trump's handling of White House records and whether he violated the Presidential Records Act and other laws related to classified information. The Presidential Records Act requires all records created by a sitting president to be turned over to the National Archives at the end of their administration.

February 18, 2022

NARA informs the Justice Department that some of the documents retrieved from Mar-a-Lago included classified material. NARA also tells the department that, despite being warned it was illegal, Trump occasionally tore up government documents while he was president.

April and May 2022

On April 7, NARA publicly acknowledges for the first time that the Justice Department is involved, and news outlets report that prosecutors have launched a criminal probe into Trump's mishandling of classified documents. Around this time, FBI agents quietly interview Trump aides at Mar-a-Lago about the handling of presidential records as part of their widening investigation.

April 11, 2022

The FBI asks NARA for access to the 15 boxes it retrieved from Mar-a-Lago in January. The request was formally transmitted to NARA by President Joe Biden's White House Counsel's office, because the incumbent president controls presidential documents in NARA custody.

April 29, 2022

The Justice Department sends a letter to Trump's lawyers as part of its effort to access the 15 boxes, notifying them that more than 100 classified documents, totaling more than 700 pages, were found in the boxes. The letter says the FBI and US intelligence agencies need "immediate access" to these materials because of "important national security interests." Also on this day, Trump lawyers ask NARA to delay its plans to give the FBI access to these materials. Trump's lawyers say they want time to examine the materials to see if anything is privileged, and that they are making a "protective assertion of executive privilege" over all the documents.

May 1, 2022

Trump's lawyers write again to NARA, and ask again that NARA postpone its plans to give the FBI access to the materials retrieved from Mar-a-Lago.

May 10, 2022

Debra Steidel Wall, the acting archivist of the United States, who runs NARA, informs Trump's lawyers that she is rejecting their claims of "protective" executive privilege over all the materials taken from Mar-a-Lago and will therefore turn over the materials to the FBI and US intelligence agencies, in a four-page letter.

May 11, 2022

The Justice Department subpoenas Trump, demanding all documents with classification markings that are still at Mar-a-Lago. At some point after receiving the subpoena, Trump asks his lawyer Evan Corcoran if there was any way to fight the subpoena, but Corcoran tells him he has to comply, according to notes Cochran took and later gave to investigators. Also after getting the subpoena, Trump aides, including body man Walt Nauta, are captured on surveillance footage moving document boxes into and out of a basement storage room -- which has become a major element of the obstruction investigation.

May 12, 2022

News outlets report that investigators subpoenaed NARA for access to the classified documents they retrieved from Mar-a-Lago. The subpoena is the first public indication of the Justice Department using a grand jury in its investigation.

June 2, 2022

As part of the effort to comply with the subpoena, Corcoran searches a Mar-a-Lago storage room and finds 38 classified documents. According to a lawsuit that the former president later filed, Trump invites FBI officials to come to Mar-a-Lago to retrieve the subpoenaed materials.

June 3, 2022

Federal investigators, including a top Justice Department counterintelligence official, visit Mar-a-Lago to deal with the subpoena for remaining classified documents. The investigators meet with Trump's attorneys, including Corcoran, and look around the basement storage room where the documents were stored. Trump briefly stops by the meeting to say hello to the officials, but he does not answer any questions. Corcoran hands over the 38 classified documents that he found. Trump lawyer Christina Bobb signs a sworn affidavit inaccurately asserting that there aren't any more classified documents at Mar-a-Lago.

June 8, 2022

Trump's attorneys receive a letter from federal investigators, asking them to further secure the room where documents are being stored. In response, Trump aides add a padlock to the room in the basement of Mar-a-Lago.

June 24, 2022

Federal investigators serve a subpoena to the Trump Organization, demanding surveillance video from Mar-a-Lago. Trump's company complies with the subpoena and turns over the footage. CNN has reported that this was part of an effort to gather information about who had access to areas at the club where government documents were stored.

August 8, 2022

The FBI executes a court-approved search warrant at Mar-a-Lago -- a major escalation of the investigation. The search focused on the area of the club where Trump's offices and personal quarters are located. Federal agents found more than 100 additional classified documents at the property. The search was the first time in American history that a former president's home was searched as part of a criminal investigation.

August 11, 2022

Trump sends a message through one his lawyers to Attorney General Merrick Garland, saying he has "been hearing from people all over the country about the raid" who are "angry," and that "whatever I can do to take the heat down, to bring the pressure down, just let us know," according to a lawsuit he later filed. Hours later, after three days of silence, Garland makes a brief public statement about the investigation. He reveals that he personally approved the decision to seek a search warrant, and that the Justice Department will continue to apply the law "without fear or favor." Garland also pushes back against what he called "unfounded attacks on the professionalism of the FBI and Justice Department."

August 12, 2022

Federal Magistrate Judge Bruce Reinhart approves the unsealing of the Mar-a-Lago search warrant and its property receipt, at the Justice Department's request and after Trump's lawyers agree to the release. The warrant reveals the Justice Department is looking into possible violations of the Espionage Act, obstruction of justice and criminal handling of government records, as part of its investigation.

August 22, 2022

Trump files a federal lawsuit seeking the appointment of a third-party attorney known as a "special master" to independently review the materials that the FBI seized from Mar-a-Lago. In the lawsuit, Trump's lawyers argue that the Justice Department can't be trusted to do its own review for potentially privileged materials that should be siloed off from the criminal probe.

September 5, 2022

In a major ruling in Trump's favor, Federal District Judge Aileen Cannon, a Trump appointee, grants Trump's request for a special master to review the seized materials from Mar-a-Lago. She says the special master will have the power to look for documents covered under attorney-client privilege and executive privilege.

September 8, 2022

The Justice Department appeals Cannon's decision in the special master case.

September 15, 2022

Cannon appoints senior Judge Raymond Dearie to serve as the special master and sets a November 30 deadline for the Brooklyn-based federal judge to finish his review of the seized materials.

October 2022

A maintenance worker drains the swimming pool at Mar-a-Lago, which ends up flooding a room where there are computer severs that contain surveillance video logs, according to CNN reporting. It's unclear if the flood was accidental or on purpose, and it's possible that the IT equipment wasn't damaged, but federal prosecutors found the incident to be suspicious.

November 4, 2022

Former Trump administration official Kash Patel testifies before the federal grand jury in the classified documents investigation. A Trump loyalist, Patel had publicly claimed that Trump declassified all the materials that ended up at Mar-a-Lago, even though there is no evidence to back up those assertions.

November 18, 2022

Garland announces that he is appointing special counsel Jack Smith to take over the investigation.

December 1, 2022

A federal appeals court shuts down the special master review of the documents that the FBI seized from Mar-a-Lago. The appeals panel rebuked Cannon's earlier decisions, writing that she essentially tried to "interfere" with the criminal probe and had created a "special exception" in the law to help Trump.

December 23, 2022

Trump attorney Timothy Parlatore testifies before the special counsel's grand jury, where he described how Trump's lawyers scoured his properties for classified materials. He later left Trump's legal team.

Late 2022 and early 2023

Trump's legal team searches four of his properties in Florida, New York and New Jersey for additional classified material. They find two more classified files in a Florida storage unit, and give them to the FBI. Around this time, Trump's team also finds additional papers with classification markings at Mar-a-Lago, and they give those materials to the Justice Department. They also turn over a laptop belonging to a Trump aide who had copied those documents onto the computer, not realizing they were classified.

Spring 2023

A string of key witnesses testify before the special counsel's grand jury in Washington, DC. This includes Trump administration officials Robert O'Brien and Ric Grenell, who handled national security and intelligence matters; Margo Martin, a communications aide who continued working for Trump after he left the White House; and Matthew Calamari Sr. and his son, Matthew Calamari Jr., longtime Trump employees who oversee security for the Trump Organization.

Mid-March 2023

In response to a new subpoena from the special counsel, Trump's lawyers turn over some material related to a classified Pentagon document that he discussed at a recorded meeting in 2021. However, Trump's team wasn't able to find the specific document -- about a potential US attack on Iran -- that prosecutors were looking for.

March 25, 2023

Corcoran, the lead Trump attorney, testifies before the grand jury in Washington, DC. This occurred after a federal judge ordered him to answer prosecutors' questions, ruling that attorney-client privilege did not shield his discussion with Trump because Trump might been trying to commit a crime through his attorneys. Corcoran later recused himself from handling the Mar-a-Lago matter.

June 2023

The first public indications emerge that the special counsel is using a second grand jury in Miami to gather evidence. Multiple witnesses testify in front of the Miami-based panel, CNN reported.

June 5, 2023

Trump lawyers meet with senior Justice Department officials -- including special counsel Smith -- to discuss the Mar-a-Lago investigation. The sitdown lasted about 90 minutes, and Trump's team raised concerns about the probe, which they have called an "unlawful" and "outrageous" abuse of the legal system.

June 7, 2023

News outlets report that the Justice Department recently sent a "target letter" to Trump, formally notifying him that he's a target of the investigation into potential mishandling of classified documents.

June 8, 2023

A federal grand jury in Miami indicts Trump in connection with the classified documents investigation, accusing him of 37 federal crimes, including illegally retaining national security documents and conspiring to obstruct justice. Trump says in a social media post that he is "totally innocent" and calls the case a "hoax." The grand jury also indicts Nauta, the Trump aide, on obstruction-related charges.

June 13, 2023

Trump pleads not guilty to 37 charges related to alleged mishandling of classified documents.

Trump's lawyers ask for a jury trial during the former president's arraignment at a federal courthouse in Miami. "We most certainly enter a plea of not guilty," Trump attorney Todd Blanche tells the judge.

July 6, 2023

Nauta pleads not guilty to multiple counts, including several obstruction and concealment-related charges, in a brief procedural hearing.

July 27, 2023

Smith brings additional charges against Trump, including one additional count of willful retention of national defense information and two additional obstruction counts, related to alleged attempts to delete surveillance video footage at Mar-a-Lago Club in summer 2022, court documents show.

New charges were also filed against Nauta and De Oliveira was added to the case.

This story has been updated with additional developments.

CNN's Jeremy Herb and Casey Gannon contributed to this report.