CHICAGO (CBS/AP) — The trial begins Tuesday for three current and former Chicago police officers charged with lying in official reports to cover up the fatal shooting of Laquan McDonald.

Officer Thomas Gaffney, former Detective David March, and former Officer Joseph Walsh have pleaded not guilty to conspiracy, official misconduct, and obstruction of justice. They’re accused of falsely claiming in police reports that McDonald attacked officer Jason Van Dyke, who shot and killed the teenager in October 2014.

A jury last month convicted Van Dyke of second-degree murder and 16 counts of aggravated battery in McDonald’s death, rejecting his claim he was defending himself when he shot McDonald 16 times. Dashboard camera video of the shooting showed McDonald was walking away from Van Dyke, and did not lunge at him as officers claimed in police reports.

March and Walsh resigned from the force after the city’s inspector general recommended they be fired for making false statements about the shooting. Gaffney has been suspended without pay.

The three officers have opted for a bench trial, meaning a judge will decide whether they are guilty of the cover-up charges.

Several community activists and pastors held a press conference Tuesday morning before the start of the trial. They said they support Special Prosecutor Patricia Brown, and believe the case is a step to reform.

Community activist William Calloway, who was a plaintiff in the lawsuit that forced the city to release the dashboard camera video of the McDonald shooting, said the fight isn’t over just yet.

“I want to make it very clear. We just don’t want to take the streets anymore. A lot of the activists, we have chose to turn our protests into policies, and the thing that we’re going to do about that, myself and so many others have made the decision to run for office to make sure that we take City Council back,” he said.

Calloway has filed to run for alderman in the 5th Ward on the South Side, against incumbent Ald. Leslie Hairston. Two other candidates also are running in that ward.

The bench trial won’t receive nearly as much media attention as Van Dyke’s, but there’s no understating the significance of using the same dashboard camera video of the shooting from the Van Dyke trial to underscore what prosecutors call the Chicago Police Department ‘s unofficial code of silence, in which officers cover for each other.

“When you go out and talk to people who are living in communities that have experienced police abuse, what really makes them feel betrayed and lose faith in the system is the officers who cover up what they’ve seen or don’t say anything,” said Christy Lopez, a Georgetown University law professor who led a federal probe of the city’s police force. “So, yes, this is very significant.”

Rev. Michael Pfleger called the unofficial police code of silence “another form of violence” in Chicago.

“We talk about the code of silence on the street, but let’s talk about the code of silence in Chicago Police Department,” he said.

Jim Pasco, executive director of the National Fraternal Order of Police, agrees that the trial is significant, but for a very different reason.

“This is going to have a chilling effect on appropriate, aggressive policing,” he said.

The charges of conspiracy, misconduct and obstruction of justice boil down to the accusation that March, Walsh and Gaffney falsified their reports about the October 2014 shooting and didn’t interview witnesses who could have provided accounts they didn’t want to record.

Prosecutors contend that Walsh, who was Van Dyke’s partner, and Gaffney, a patrolman, wrote among other things that McDonald assaulted Van Dyke. Gaffney claimed Van Dyke and other officers had been injured. Further, Walsh supported Van Dyke’s claim that McDonald lunged at the two of them with a knife and, even after bullets knocked McDonald down, he “attempted to get up while still armed with a knife.”

None of these details was apparent on the dashcam video that captured the shooting and has been shown on news shows countless times since a judge ordered the city to make it public a year after the shooting.

Prosecutors say March — a detective who investigated the shooting and who along with Walsh has since left the department — not only cleared Van Dyke of any wrongdoing by saying the video matched witness accounts, but also told another officer to include false information in her report. Gaffney remains on the force but has been suspended.

While no other officers have been charged, the special prosecutor, Patricia Brown Holmes, said it is clear that others on the force, including brass, wanted Van Dyke to be cleared.

“We should be applauding him, not second guessing him,” wrote a sergeant identified only as March’s supervisor in an email to a lieutenant. Prosecutors contend police also shooed witnesses from the scene, and that the defendants and others met at a station to “conceal the true facts” of the shooting.

Not surprisingly, attorneys for the three dismiss any talk of conspiracy.

“The whole indictment is a sham, based not on evidence, but on politics,” March’s attorney, James McKay, said at a recent hearing.

Lopez, the law professor, said a conviction would send a powerful message to police officers all over the country that, “You can be held accountable, even if you didn’t pull the trigger.”

But Robert Weisskopf, a retired Chicago police officer and former president of the lieutenants’ union, said that message has already been sent.

“Are you really going to work that hard and do anything more than the bare minimum if you think you could go to prison, lose your mortgage, have protesters in front of your house?” he asked. “They’re going to say, ‘Screw that.'”

Others say the lessons of the shooting are less about politics and changing attitudes toward police, and more about the growing role of technology in policing.

“You have a partner who screws up, you cover for him. That’s not going to change,” said Terry Ekl, a lawyer whose lawsuit in a videotaped beating of a female bartender by an off-duty Chicago police officer led the jury to conclude there was a code of silence in the department. “The use of the video, that’s what’s going to be a strong deterrent to condoning misconduct.”

Chicago’s second trial in a matter of weeks to rely on the same explosive video will, he said, only remind everyone of that.

(© Copyright 2018 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)