(CNN) Investigators are struggling to build a federal murder case regarding fallen US Capitol Police officer Brian Sicknick, vexed by a lack of evidence that could prove someone caused his death as he defended the Capitol during last month's insurrection.
Authorities have reviewed video and photographs that show Sicknick engaging with rioters amid the siege but have yet to identify a moment in which he suffered his fatal injuries, law enforcement officials familiar with the matter said.
Soon after Sicknick died on January 7, prosecutors in Washington opened a federal murder investigation, dedicating a team inside the US attorney's office to build out a case, authorities have said.
To date, little information has been shared publicly about the circumstances of the death of the 13-year veteran of the police force, including any findings from an autopsy that was conducted by DC's medical examiner.
In a statement the day after the insurrection, Capitol Police said that Sicknick had been "injured while physically engaging with protesters" and collapsed as a result of his injuries sometime after returning to his office. He died the next day in a local hospital.
Sicknick will lie in honor at the Capitol beginning Tuesday evening, Congressional leaders announced late last week, bestowing on him a sacred tribute that's typically reserved for dead American political leaders.
In Sicknick's case, it's still not known publicly what caused him to collapse the night of the insurrection. Findings from a medical examiner's review have not yet been released and authorities have not made any announcements about that ongoing process.
According to one law enforcement official, medical examiners did not find signs that the officer sustained any blunt force trauma, so investigators believe that early reports that he was fatally struck by a fire extinguisher are not true.
One possibility being considered by investigators is that Sicknick became ill after interacting with a chemical irritant like pepper spray or bear spray that was deployed in the crowd. But investigators reviewing video of the officer's time around the Capitol haven't been able to confirm that in tape that has been recovered so far, the official said.
The case could also be complicated if Sicknick had a preexisting medical condition. It could not be learned if he did.
A spokeswoman for the US Capitol Police declined to comment for this story citing the ongoing investigation.
As part of the investigation into the riots, prosecutors have charged several with assault against a federal officer stemming from violent episodes at the Capitol, separate from the one involving Sicknick.
In court documents, FBI agents have laid out elements of the attacks faced by officers in alarming detail. More than 100 police officers were injured in the melee, including at least 15 officers who required hospitalization, according to court documents.
One man can be seen in surveillance video striking uniformed officers from the Capitol Police and the DC's Metropolitan Police Department with a metal baseball bat, according to an FBI affidavit.
Another man allegedly threw a fire extinguisher at a group of police officers, striking three in the head, an affidavit says.
Sicknick's case, however, will be without resolution when he arrives to the capitol in a ceremony Tuesday night before a viewing period for members of his police force.
A congressional tribute will be held on Wednesday morning, according to information released by House Speaker Nancy Pelosi and Senate Majority Leader Chuck Schumer.
Because of the pandemic, the ceremony will be closed to the public. Later that day, Sicknick will depart the Capitol ahead of an interment at Arlington National Cemetery.
"The U.S. Congress is united in grief, gratitude and solemn appreciation for the service and sacrifice of Officer Brian Sicknick," Pelosi and Schumer said in a joint statement. "May this ceremony and the knowledge that so many mourn with and pray for them be a comfort to Officer Sicknick's family during this sad time."
In an email Tuesday, a spokeswoman for the Sicknick family declined to offer further comment about the status of the investigation into the officer's death.
The Sicknick family has thanked the congressional leaders and expressed appreciation "to the millions of people who have offered their support and sympathies during this difficult time."
"Knowing our personal tragedy and loss is shared by our nation brings hope for healing," the family said in a statement released over the weekend.