Opening statements began Tuesday in the trial of three former upstate New York prison guards charged in the deadly beating of inmate Robert Brooks last December at Marcy Correctional Facility. 

Mathew Galliher, Nicholas Kieffer and David Kingsley are charged with murder and first-degree manslaughter in the Dec. 9 beating that was captured, in part, on body-worn cameras. They were among 10 corrections officers indicted on murder or lesser charges in the case.

Onondaga County District Attorney William Fitzpatrick told jurors in his opening statement that the guards took part in an act of "sheer, unimaginable brutality" and that each defendant was "intimately involved."

"They no longer were corrections officers. They were a gang," Fitzpatrick said. "They took turns — collectively and individually — of punching him, kneeing him, pepper spraying him, choking him, pinning him down, cuffing his legs."

Brooks, 43, had been serving a 12-year sentence for first-degree assault since 2017 and was transferred to Marcy from a nearby lockup that night. He was dead within an hour, before he even unpacked, Fitzpatrick said.

The videos, which triggered widespread outrage, show Brooks in handcuffs as the officers strike him in the chest with a shoe, lift him by the neck and drop him.

Fitzpatrick, the special prosecutor, said Brooks died from the massive beating that broke a bone in his neck, ripped his thyroid cartilage and bruised several internal organs. He also died as a result of repeated restrictions to his airways, which caused brain damage, and choking on his own blood.  

Here's what the defense has to say

Defense attorneys told jurors that prosecutors will not be able to prove their clients acted with malice or depraved indifference to human life, as the charges allege. The attorneys asked jurors to take careful note of their clients' specific actions that night.

"The prosecution is attempting to tie Nicholas Kieffer to the actions of others, suggesting to you that he is somehow responsible via association," said his attorney, David Longeretta. Kieffer applied a "minimum amount" of pepper spray to Brooks to gain his compliance, the lawyer said.

Galliher's attorney, Kevin Luibrand, said his client has been charged with murder, in large part, for shackling Brooks' legs to keep him from kicking.

"Mathew Galliher didn't harm Robert Brooks. He didn't hit him, he didn't strike him, he didn't encourage others to strike him, he didn't deny him medical care," Luibrand said. "He didn't do anything that contributed to the death of Robert Brooks."

Several prison staffers charged in Brooks' death

Fitzpatrick said Brooks was beaten three separate times as soon as he arrived at the prison, the last being the fatal beating in the infirmary caught on the silent body-camera footage.

Six guards indicted in February have since pleaded guilty. Three more employees have agreed to plead guilty to reduced charges and are cooperating with the special prosecutor.

A fourth corrections officer is scheduled to go on trial for second-degree manslaughter in January.  

"I want them all to get prosecuted. It was murder, there's no other word for it. They murdered my father, they robbed him from me. He doesn't get to come home," Brooks' son, Robert Brooks Jr., told CBS News after his death. 

Fitzpatrick is also prosecuting guards in the fatal beating of Messiah Nantwi on March 1 at the Mid-State Correctional Facility, which sits across the street from Marcy. Ten guards were indicted in April, including two who are charged with murder, in Nantwi's death.

Both prisons are about 180 miles northwest of New York City.