In an eight-page letter expressing “grave concern,” the family of Tamir Rice, the 12-year-old boy shot and killed by a Cleveland police officer last November, called for the local prosecutor who is overseeing the investigation of the two officers involved to step aside and appoint a special prosecutor.

The letter, obtained by The Washington Post and addressed to Cuyahoga County Prosecutor Timothy McGinty from Rice family attorney Jonathan Abady and his co-counsel, argues that McGinty has unnecessarily delayed the grand jury process and decried his decision last week to release two expert opinions on the shooting that suggested the shooting was justified.

“The delay in presenting this case to a grand jury, the decision to retain pro-police ‘experts’ and release their reports to the media on a Saturday night over a holiday weekend… and the obvious shortcomings of the reports themselves have contributed to make the Rice family feel that your office is not committed to securing an indictment in this case,” attorneys for Tamir Rice’s mother wrote in the letter dated Oct. 16. “It now appears that the grand jury presentation will be nothing short of a charade aimed at whitewashing this police killing of a 12-year-old child.”

The calls for McGinty to step aside are expected to be repeated Friday morning, when Rice’s mother and lawyers will hold a news conference in Cleveland.

“We respectfully request that you recuse yourself from this case and that a special prosecutor be appointed to investigate and prosecute this matter,” Rice’s family’s attorneys wrote to McGinty. “That is the only way that Tamir’s family, the community in Cleveland, and the nation as a whole will have any faith in the process.”

Tamir Rice was shot on Nov. 22, 2014, as he played with a toy gun in a park near his home on the west side of Cleveland. Officers were called to the park after a resident reported that a young man was waving what appeared to be a gun but was ‘probably fake.’ When officers Tim Loehmann and  Frank Garmback arrived, they pulled their patrol car onto the snow-covered grass of the park, within seven feet of Tamir.

In video that captured the shooting, Loehmann is seen exiting the passenger side of the vehicle. In less than two seconds, Tamir was shot in the chest and was on the ground.

The shooting came as the nation was locked in an emotional examination of police killings of unarmed black men. Just days after Tamir was killed, a grand jury in Ferguson, Mo., would side against indicting white officer Darren Wilson for the death of Michael Brown, a black teenager. Weeks after that, a Staten Island grand jury would decline to charge a New York City police officer in the death of Eric Garner.

Of the incidents of police officers killing unarmed black men last year that gained national attention and scrutiny, Tamir Rice’s death remains the only case in which a decision about whether the officers involved will be charged with a crime has yet to be rendered.

The shooting was initially probed by the Cuyahoga County Sheriff’s Office, which then gave the evidence that it had gathered to McGinty’s office.

On Saturday, McGinty released the expert opinions of two former police officials, commissioned as part of his investigation, both of whom concluded that the shooting was legally justified.

“It could be argued that the officers enhanced that risk by entering the park and stopping their vehicle so close to a potentially armed subject,” wrote retired FBI special agent Kimberly A. Crawford. “However, this type of ‘armchair quarterbacking’ has no place in determining the reasonableness of an officers [sic] use of force.”

Since the reports were released, the Rice family’s attorneys have questioned the credentials of both Crawford and Lamar Sims, a prosecutor from Colorado who conducted the second expert review — noting that Crawford’s previous analysis of a police shooting was rejected by the Department of Justice and that Sims had previously done a television interview in which he appears to side with the officers in Tamir Rice’s shooting.

“There is no question that the ‘experts’ you selected were biased,” Rice family’s lawyers wrote to McGinty.

McGinty has not said when he will present the case to a grand jury, and said in a statement last week that he plans to continue releasing parts of the investigation as he receives them.

“This approach by our office has ended the protocol of total secrecy that once surrounded the use of deadly force by law enforcement officers,” McGinty said. “When a citizen is purposefully killed by police, the results of the investigation should be as public and transparent as possible.”