SAN JOSE — Shortly after jailers entered his Santa Clara County Main Jail cell, Michael Tyree started “screaming for his life,” a fellow inmate testified Thursday in the trial of three guards charged with beating him to death.

Juan Perez, 33, the first inmate witness to take the stand in the closely watched murder trial, said he had a partial view into Tyree’s cell from his own and also could hear some of what was said.

“They went to Tyree’s cell, and it was all bad,” Perez testified. “It took like 15 seconds before he started screaming for his life.”

Perez said it sounded like someone getting beaten down, that he heard “Help! Help! Please stop!” and “Boom! Boom!” Then it was silent.

As the guards were leaving, “They turned off his light,” said Perez, which he said they hadn’t done during any other cell search that night. According to Perez, one of the guards then said, “I don’t want to hear another word.”

The guards — Matthew Farris, Rafael Rodriguez and Jereh Lubrin — have pleaded not guilty to second-degree murder in the Aug. 26, 2015, death of Tyree, and to assaulting another mentally ill inmate, Juan Villa, earlier that evening. Tyree’s death touched off outrage and spurred an ongoing effort to improve conditions and oversight of the county jails.

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If convicted, Farris and Rodriguez, both 28, and Lubrin, 30, could be sentenced to life in prison. They are free on $1.5 million bail each and on paid administrative leave. The prosecution doesn’t have to prove they intended to kill Tyree, only that his death was caused by their dangerous conduct and reckless disregard for human life.

At the time of Tyree’s death 18 months ago, Perez was in jail on federal immigration charges. He had previously served 11 years for a gang enhancement, gun possession and money laundering, but claimed he has since turned his life around, and has a job and children.

Lawyers for the guards are set to grill him Monday about his criminal history and motives for testifying.

But Perez has consistently given the same account, prosecutor Matt Braker suggested. In a jail recording of a phone conversation that Braker played for the jury Thursday, Perez told the same story to his best friend’s mother. He also had her call a detective he knew to report the incident.

“These cops killed an inmate here,” he said over the phone to his mother’s friend, adding, “All you heard was screaming. They’re trying to cover it up.”

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Perez said Lubrin returned to the unit later that night to do welfare checks. The guard opened Tyree’s cell door, saw him lying on the floor and nudged his unresponsive, feces- and vomit-smeared body with his foot a couple of times, Perez said. He then moved on to another cell, asking an inmate there, “Does Tyree s— himself all the time?”

But it took about five minutes for the guards to summon help for Tyree, according to a top-ranking jail official who testified earlier in the day Thursday.

Capt. Tom Davis testified that the guards should have immediately called for medical help after noticing 31-year-old Tyree lying on the floor of his cell shortly after midnight on Aug. 27, 2015.

Davis said guards are supposed to take immediate action by making radio notification of an inmate down, and then administer aid, including CPR.

Lawyer Matt Pavone, representing Rodriguez, attributed the delay to the guards’ relative inexperience — they had worked in the jail only two to three years.

Farris’ lawyer, Bill Rapoport, suggested the guards are trained not to open a cell door and render aid without another guard present for safety reasons. However, there is no record of an immediate radio call for medical help even after the second guard entered the pod.

Judith Odbert, the lawyer representing Lubrin, also questioned how Davis could possibly know when the guards realized Tyree was having a medical issue as opposed to being asleep on his cell floor. However, the floor and walls of Tyree’s cell — as well Tyree himself — were smeared in vomit and feces, signaling a possible medical problem.

Braker told jurors at the trial’s opening earlier this week that Tyree died from an abuse of power by his jailers and that he couldn’t have caused the injuries that killed him by himself. The guards’ attorneys countered that evidence will show Tyree fatally injured himself and was suicidal.

The trial is set to resume Monday.