SAN JOSE  (AP) — Three Northern California jail guards accused of beating to death a mentally ill inmate have also been charged with assaulting another prisoner, prosecutors announced Tuesday.

The three Santa Clara County jail guards were arrested last week after Michael Tyree, 31, was found beaten to death his cell shortly after midnight Aug. 27.

The three guards were formally charged with murder Tuesday. Prosecutors also charged the three with assaulting inmate Juan Villa the night Tyree died.

Guards Jereh Lubrin, Rafael Rodriguez and Matthew Farris appeared in court Tuesday, but they didn’t enter a plea. They remain jailed in Alameda County without bail.

Rodriguez denied involvement in Tyree’s death in a jailhouse interview with the San Jose Mercury News published Sunday.

“These men violated the law, human dignity, and the job that they were sworn to do,” District Attorney Jeff Rosen said

According to court records, after a fresh clothing exchange the guards were searching for extra garments inmates may have hidden in their cells. The three guards are alleged to have entered Villa’s cell and hit him in the head and twisted his arm after the inmate got into a dispute with another prisoner during the clothing exchange.

The guards made their way to Tyree’s cell, and one of the guards was heard ordering the inmate to stand up, according to court documents.

Investigators say the schizophrenic Tyree had a “distinctive voice” that other inmates heard screaming “I’m sorry! I’m sorry! Stop!”

Investigators say Tyree had a few hours earlier refused to take his medication, requiring the attention of the guards.

Inmates told police they could hear Tyree screaming for several minutes and also heard “sounds of thumping, wall banging and what sounded like blows to a person’s body.”

Tyree was found dead in his cell around midnight Aug. 27. Investigators say Tyree died from internal bleeding and suffered significant liver and spleen damage.

Santa Clara County Sheriff Laurie Smith said last week that the three guards were the only people with access to Tyree’s cell the night he died.