Chicago (CNN) At least three witnesses to the Laquan McDonald police killing were questioned for hours, threatened by officers and ordered to change their accounts to match the official Chicago police version of the shooting, the attorneys for the teen's estate say.

The allegations are contained in more than 3,000 pages of recently released documents related to the case. The attorneys also allege that police officers up the chain of command fabricated witness accounts to support the way officers at the scene described the October 20, 2014, shooting as justified.

"It's not just the officers on the street," attorney Jeffrey Neslund told CNN. "It's a lieutenant, a sergeant and detectives -- and the lengths they went to justify what simply was not true."

CNN contacted Neslund and Michael Robbins after the city released the documents to the news media in response to multiple Freedom of Information Act requests.

Asked if they stood by the accusations they made in letters to the city's corporation counsel last March, Robbins said, "Absolutely."

"You have a false narrative put out by police," he said, "outright lies to cover up an illegal shooting, corroborated by other officers."

CNN asked the Chicago Police Department for a reaction to the allegations. "This is the first time I'm hearing of that allegation," spokesman Anthony Guglielmi said. "But this is why we want an independent investigation to look at every fact. But unfortunately, we're not able to comment on any specific incident."

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CNN legal analyst Joey Jackson said the allegations made by Neslund and Robbins are a significant development.

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"In the event that it's true, you have multiple underlying charges: intimidating witnesses, falsifying public records and you have a conspiracy that more than one person was involved," Jackson said. "This has the potential of taking down a lot of people in that police department."

"The issue then becomes how high of a level does this go: Who knew about this?"

Mayor Rahm Emanuel 's office declined to make the mayor available for an interview but issued the following statement to CNN regarding these allegations:

"The police actions surrounding this shooting are under investigation by the U.S. Department of Justice for possible criminal charges, and by the city's Inspector General for possible disciplinary action. The public deserves answers to a number of important questions in this case, and we eagerly await the findings of those investigations."

Police Officer Jason Van Dyke, 37, was charged with first-degree murder in late November in the killing of McDonald, 17. The officer pleaded not guilty in December and is free on bail. His attorney has said he feared for his life before he opened fire, shooting the teen 16 times.

At least five officers at the scene, including Van Dyke's partner, backed up his account that McDonald lunged at him. "When McDonald got to within 12 to 15 feet of the officers, he swung the knife toward the officers in an aggressive manner," Van Dyke's partner said in an official police report.

But a police dashcam video shows McDonald walking down South Pulaski Road with a knife in his hand, heading away from officers. The city fought the video's release for 13 months. Van Dyke was charged just hours before it was made public.

Attorneys: Witness said an officer was 'going to get me'

Attorneys Robbins and Neslund were seeking a settlement with the city on behalf of McDonald's family when they requested witnesses' statements to police. They personally interviewed three witnesses. They found that the accounts police provided did not match what witnesses told the lawyers.

These were the accounts the lawyers say witnesses gave to them:

A motorist and his son who witnessed the shooting said a uniformed officer told the man "to get out of there immediately, to drive off or be arrested," Robbins told CNN.

"This is somebody who is an occurrence witness to a fatal shooting," Robbins said. "Nobody asked him, 'What did you see?' "

Another witness, a truck driver who was at a nearby Burger King, told the attorneys that he and two other witnesses, a woman and her friend who both saw the shooting, were put in police cars, taken to a station and interviewed for hours in separate rooms.

"He kept describing it and he said the police were visibly angry with him and arguing with him about what happened, saying, 'That's not what happened,' " Robbins said. "He'd say, 'Well, that's what I saw.' They said, 'No, you're wrong.' "

At one point, the trucker told police he needed to get back to work for a 6 a.m. shift, according to Robbins. "The police said, 'We don't give a f--- about your truck. Let's go through this again,' " Robbins said.

In their letters to the city, the lawyers describe the account of the woman taken to the same station with the truck driver. They did not interview her but say she spoke to McDonald's family and told them the teen was "trying to run away from (police)," Neslund wrote.

The woman, Neslund alleged, was so appalled when shots rang out that she yelled at Van Dyke: "Stop shooting."

"There's a reason they kept us there 'til 4 a.m.," the woman told McDonald's family, according to Neslund. "One officer said he was going to get me."

When the witnesses refused to change their statements, Robbins wrote, "the investigating officers simply fabricated civilian accounts in the reports."

The woman, her friend and the truck driver were released after about six hours of questioning when one of them asked for an attorney, Robbins said.

The threats and intimidation of the female witness, Neslund alleged, continued in the days after she first spoke with police.

'We were told there were no witness statements'

In response to the attorneys' requests for witness statements, the city allowed them to examine summaries of witness accounts. Those summaries said five people in the vicinity of the shooting were interviewed by police, but none of them saw the shooting. Two said they "did not see or hear anything," according to the official police records. Another witness summary said a third person heard gunshots and then saw McDonald was "lying in the street." The two others said they saw McDonald being chased by police, but did not see the shooting, according to the official police version.

The names of people interviewed by police are redacted in the publicly released documents. One of the redacted names is listed as a "self-employed truck driver."

Neslund and Robbins confirmed that the trucker they interviewed is the person in those redacted documents. When the attorneys first approached him and told him the police records say he didn't see the shooting, he told them that was not true.

"The truck driver says he did tell police, that it was like an execution," Robbins said. "What he described was what we saw in the video."

CNN asked the attorneys to put the network in touch with the witnesses for interviews. They declined.

The Police Department's witness summaries were submitted into the record on March 15, 2015, nine days after the attorneys sought information and nearly five months after the shooting. The document was signed off and approved by a Chicago police lieutenant.

"We saw these (summaries) by the three witnesses who were interviewed at the station -- that police say they didn't see anything. We said, 'Where's the witness statements?' We were told there were no witness statements," Robbins said.

""Significantly in our view," Robbins said, "of these three witnesses -- the truck driver, the woman and her companion -- none of them were asked to sign a statement."

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A federal grand jury has been investigating the shooting for months, including looking into possible obstruction of justice charges by police officers. Neslund and Robbins, who both were once Cook County prosecutors, said it is their understanding the truck driver has testified before the grand jury.

The U.S. Attorney's Office for the Northern District of Illinois declined to comment about specifics related to the grand jury. The office would say only that a joint federal and state investigation into the shooting continues and that authorities are "conducting a thorough investigation into the circumstances of the shooting."

"We won't be saying anything in addition to that," said Joseph Fitzpatrick, an assistant U.S. attorney.

Other officers still working

As of now, no other officer connected to the McDonald case has faced any charges, and they remain on their beats.

The Chicago Police Department has said it is awaiting the conclusion of the Justice Department inquiry and other investigations, but will take swift action if "any officer participated in any wrongdoing."

Corporation counsel of the city of Chicago Stephen Patton and his deputy, Thomas Platt, have yet to respond to a CNN request about whether they took any steps to investigate the allegations raised by the estate attorneys.

Neslund and Robbins first met with Patton and Platt in March on the settlement negotiations.

The estate originally sought $16 million, $1 million for every bullet that struck McDonald. A settlement of $5 million was reached in April.

In a demand letter dated March 6 and written after he'd seen the dashcam video, Neslund said:

"This case will undoubtedly bring a microscope of national attention to the shooting itself as well as the city's pattern, practice and procedures in rubber-stamping fatal police shootings of African-Americans as 'justified.' "

"I submit this particular shooting can be fairly characterized as a gratuitous execution ... as well as a hate crime."

The case stayed largely out of the spotlight until a judge in November ordered the video be made public. Just as Neslund had predicted, it shocked the city and nation.