(CNN) On the fourth anniversary of Eric Garner's death Tuesday, his mother stood on the steps of City Hall and demanded action from authorities.
Just a day earlier, the New York Police Department said in a letter to the Justice Department that it would start disciplinary proceedings against officers in Garner's death as early as September.
Garner's mother, Gwen Carr, called the letter a "smokescreen" and demanded to know why the NYPD won't act sooner.
"(Mayor Bill) de Blasio -- why September? Why September?" Carr asked in front of supporters. "When you could have moved in April and May and June and now it's July. Let's move today. We have to get action on this case."
A deputy press secretary for de Blasio, Olivia Lapeyrolerie, told CNN, "We share the family's frustration. The process has taken too long, and the NYPD is no longer waiting for the DOJ to begin disciplinary proceedings."
Garner died after police attempted to arrest the 43-year-old father of six for allegedly selling cigarettes illegally in Staten Island.
Video shows officer Daniel Pantaleo tackling Garner from behind using a department-banned chokehold. Pantaleo has remained on NYPD's payroll.
In its letter, the NYPD said it will go ahead with disciplinary proceedings against Pantaleo as early as September 1 if the Justice Department doesn't say by then whether it will press federal charges.
NYPD Deputy Commissioner of Legal Matters Lawrence Byrne wrote that the police department delayed starting the disciplinary process at the request of the DOJ so it would not impact the DOJ investigation.
"However, based on our most recent conversations, it has become clear that a definite date by which time a final decision by the US DOJ will be rendered in this matter cannot be predicted," Byrne wrote.
Byrne added that due to the "extraordinary passage of time," any further delay in starting the internal proceedings "can no longer be justified."
The "internal administrative disciplinary proceedings" would determine if Pantaleo would face administrative discipline. Those proceedings would not determine criminal charges. An attorney for Pantaleo declined to comment on the case Tuesday.
The NYPD said it would also launch disciplinary proceedings against Pantaleo's supervisor, Sgt. Kizzy Adonis, who is still on active duty.
In a statement, the Department of Justice said: "As officials at the Department of Justice informed Mr. Byrne this spring, the New York Police Department may move forward with its disciplinary proceedings. Mr. Byrne's letter does not have any bearing on the decision-making timeline at the Justice Department, and the Department cannot comment further at this time."
An NYPD official disputed the Justice Department statement, saying the city was not informed by the Justice Department that it could go ahead with proceedings this spring.
A Justice Department official added Tuesday that lawyers in the Civil Rights Division had phone conversations with the NYPD in 2017 and 2018 and consistently made clear they wouldn't object to the department moving forward with disciplinary proceedings. However, lawyers did say in September 2017 that if the NYPD planned to move forward, it should consider holding off on discovery while the criminal investigation was pending, as is typical when evidentiary issues overlap, the source explained.
An NYPD official said the department has had several phone conversations with the DOJ "going back to the beginning of 2015," and under two different administrations, when the department was told not to go forward with its internal proceedings until the DOJ's investigation was completed.
The NYPD said it filed discipline charges against Adonis, but that the DOJ urged the department to pause the process until the criminal investigation was completed.
Patrick J. Lynch, president of the Patrolmen's Benevolent Association, which represents Pantaleo, said it agrees the DOJ should move to close the officer's case "and put an end to what has been a highly irregular fishing expedition by those seeking an indictment at all cost."
"However, that should not trigger a race by the NYPD to reach a predetermined outcome in its own disciplinary processes," Lynch said.
Lynch said Pantaleo "is entitled to due process and an impartial consideration of the facts. If that is allowed to occur, we are confident that he will be vindicated and will finally be able to move forward."
A representative from Adonis' union, the Sergeants Benevolent Association, could not be reached.
A 2014 grand jury in New York declined to indict Pantaleo, a Staten Island officer who is white. Garner was black.
The Civilian Complaint Review Board, known as CCRB, the independent entity that would oversee the case against officers in administrative proceedings, recommended disciplinary action against Pantaleo in 2017 for using a chokehold.
"The CCRB's Administrative Prosecution Unit stands ready to prosecute Officer Pantaleo as it does in cases in which the board substantiates misconduct against a member of the NYPD and recommends charges and specifications," CCRB Chairman Fred Davie said in a statement.
In a news conference shortly after the NYPD's letter to the DOJ was released, the Rev. Al Sharpton criticized the DOJ's slow response to the case.
"It seems the height of hypocrisy that the DOJ is going to reopen the case of Emmett Till that's 60 years old and will not move forward on a case that's four years old," the Rev. Al Sharpton said Monday while standing with Carr, Garner's mother.
At Sharpton's event Monday, Carr asked that all officers involved with the death of her son be fired, adding that she still has not been told the names of the other officers.
"Please move forward on this and fire those officers -- all of them, not just Pantaleo. We want them all fired," she said. "They all were misbehaving on that day."
The mayor, in an appearance Monday evening on CNN affiliate NY1, said the NYPD is doing "exactly the right thing here."
"I think anyone who's frustrated with the aftermath of the death of Eric Garner, I understand that 100%," he said on the program "Inside City Hall." "This has not been the way things are supposed to be because of the reality of the Justice Department."