From [HERE] Video that could have exonerated an imprisoned man beaten by former Metra police officers was deliberately buried by the agency for 20 months, a lawsuit states -- a claim Metra denies.

Attorneys for D'Nardo Mack said Metra turned "a blind eye" when their client was wrongfully arrested on a charge of attacking a Metra officer, who on Monday was charged with aggravated battery in the case.

Metra leaders said they fully cooperated with authorities and the video had been recorded over by the time it was required.

Prosecutors say former officer David Robertson, 50, of Matteson struck Mack in the face unprovoked, then pepper-sprayed and pummeled him with a baton Jan. 15, 2015, at Millennium Station.

A copy of that video that Robertson filmed with his phone while watching with other officers, who made jokes throughout, bears that out and caused Cook County officials to drop the charges against Mack, a 45-year-old Chicagoan.

Robertson's attorneys said he is innocent. Meanwhile Metra leaders said they were appalled by the actions of the officers and took immediate action to investigate when contacted by the Cook County state's attorney's office in September 2016.

Mack's attorneys claim Metra "deliberately withheld video evidence of the incident" although prosecutors and defense attorneys asked Metra to produce it.

Metra spokesman Michael Gillis said "our system is set to record over itself after 30 days. By the time we were able to reply to the office of the Cook County state's attorney's request for the original recording, it had been overridden.

"We were unaware that Robertson had made a copy of the original recording until the office of the Cook County state's attorney notified us in November 2016. We immediately began our internal investigation at that time," Gillis said.

State's attorney's office spokesman Robert Foley said that "Metra has fully cooperated with our investigation."

The lawsuit is in the process of being settled.

Mack's attorneys said his civil rights, including the provision against unreasonable seizure, were violated and he needed to be taken to the hospital after the beating.

Mack suffered physical and mental pain, emotional distress, disability and "loss of a normal life," the lawsuit states.

After a scathing assessment of its police force, Metra instituted reforms and hired a new police chief in 2014.

"As part of the reform of the department, since that time we have enacted a procedure to review video of an arrest if it involved a physical confrontation and occurred in an area where video is available," Gillis said Tuesday.

In November 2016, Metra officials said, Robertson and a second officer who participated in the arrest were suspended without pay. Both were fired in December. A third officer who took part in beating the man was dismissed before the internal investigation.

Of the officers who were heard laughing in the background on the video, two were disciplined and two no longer work at Metra, Gillis said.

Robertson testified the other man started the fight and "rushed" him, then reached into his right pants pocket, which led the police officer to pull out the pepper spray.