SAN JOSE — Expert testimony Thursday on behalf of three Santa Clara County jail guards charged with a mentally ill inmate’s murder suggested he injured himself in a possible suicide attempt and was further hurt by aggressive efforts to revive him.

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Jail guards’ murder trial: Defense grills witness who says he heard Michael Tyree screaming “for his life” The defense and prosecution agree that Michael Tyree died in August 2015 from blunt force trauma in his solitary cell. Prosecutor Matt Braker claims Tyree, whose liver and spleen were lacerated, died from being repeatedly struck and possibly kicked by the three correctional deputies.

The guards — Jereh Lubrin, Matthew Farris and Rafael Rodriguez — have pleaded not guilty to second-degree murder of Tyree and to assaulting another mentally ill inmate, Juan Villa, shortly before they got to Tyree’s cell. 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. Tyree’s death touched off outrage and spurred an ongoing effort to improve conditions and oversight of the county jails.

The guards’ lawyers argue Tyree injured himself when he either jumped or fell in his cell, perhaps by landing on a hard surface like the lip of the cell sink.

Dr. Judy Melinek, a pathologist at the Alameda County coroner’s office and private consultant who formerly worked in Santa Clara County, testified Thursday she researched liver lacerations caused by aggressive CPR and injuries to the spleen from a fall. She used a life-sized anatomical model to explain the organs’ location to the jury.

Melinek said she would expect to see broken ribs over Tyree’s spleen if he were struck with an object like a fist or baton, but he had no cracked ribs or bruising there.

Melinek also testified that a chevron-shaped injury found on Tyree’s body is similar in size and shape to edge of his cell sink. And she testified that a cut found above Tyree’s eyebrow that barely bled was not the result of being assaulted but consistent with a person falling down while dying.

And Melinek testified that Tyree’s liver was lacerated by aggressive and perhaps prolonged CPR by people who are not medical practitioners and had their hands in the wrong place.

Melinek also cast doubt on the thoroughness of the investigation by the Santa Clara Clara County Medical Examiner-Coroner’s Office. Under questioning by attorney Judith Odbert, who represents Lubrin, investigator Chris Garcia acknowledged he never stepped inside inmate Michael Tyree’s one-man cell after he was found dead.

Garcia testified he stayed out of the cell to avoid contaminating a potential crime scene and because Tyree’s vomit and feces in the cell posed potential biological hazards.

But Odbert suggested that writings on Tyree’s cell walls such as “Rest in Peace” and “Forgive my sin” were not taken into consideration when the coroner determined the manner of death was homicide rather than suicide. There also was an open Bible on the table as well as an unspecified novel that were not examined.

Earlier in the trial, two inmate witnesses for the defense testified they heard Tyree vow to kill himself.

“It is a grave mistake to interpret the cause of death just from the body,” Melinek said. “You need to go to the scene or at least look at photos. You can’t interpret in a vacuum.”

Prosecutor Matt Braker contends there is no proof Tyree wrote the scrawls on his cell wall and also that they appear to make little sense. Among them: “I’ll reset time when I reach my goal and put the world back the way it is.”

Braker has also called inmates and former inmates to the stand who testified they heard both Tyree and another inmate scream in pain that night.

The county medical examiner’s pathologist has testified that the blunt force trauma to Tyree was equivalent to getting hit by a car.

The jury also has seen sadistic-sounding text messages, principally from Farris, that Braker says show Farris and other guards enjoyed abusing inmates, particularly so-called “J-cats,” jail lingo for mentally ill.

But Odbert and the other guards’ lawyers, Matt Pavone and Bill Rapoport, called to the stand some of their clients’ colleagues, who characterized the texts as sarcasm or banter.

Earlier this week, the defense tried to get Judge David A. Cena to essentially reduce the murder charge. The guards’ attorneys wanted Cena to exclude first-degree murder from the jury’s eventual consideration and limit the panel to weighing only second-degree or manslaughter.

Unless there are special circumstances involved in a homicide like kidnapping, a murder charge never specifies whether the act was first-degree or second-degree. First-degree is punishable by 25 years to life; second-degree murder, 15 to life.

On Thursday, Cena rejected the defense request, saying that even if the prosecution can’t prove the killing was intentional, the jury is allowed to consider first-degree murder under the “felony murder rule.”

Under that rule, if offenders kill (regardless of their intent) during the commission of certain felonies, they can be convicted of first-degree murder. In this case, the prosecution claims the felony was a burglary, meaning the guards unlawfully entered the cell to make Tyree suffer for mouthing off to a nurse earlier in the evening.