MORGAN HILL — A judge Tuesday declared a mistrial in the case of a former Santa Clara County jail guard accused of kicking an inmate in the head while he was lying face down on his cell floor with two other guards over him.

Santa Clara County Superior Court Judge Edward F. Lee dismissed the jury after it deadlocked 11-1 in favor of acquitting former correctional officer Thanh Hung Tri on assault charges.

There were signs early on that the jury would fail to reach a consensus. On the first full day of deliberations Monday, the jurors left early after reporting to the judge that they were stuck 11 to 1 on the second charge — assault under color of authority. The lone holdout, a man, had sworn at one of the other jurors, a note to the judge said.

Related Articles Another Santa Clara County jail guard in trial on excessive force charge By midday Tuesday, that juror had sent a note to the judge saying he felt like he was being held hostage. He was the only juror who voted to convict Tri of the 2013 alleged assault; the other 11 voted to acquit him.

It was not immediately clear whether the District Attorney’s Office will retry the case, but the decision is often based partly on how many jurors voted to convict. The case was never considered particularly strong; prosecutors in 2014 initially declined to file charges against Tri “due to lack of jury appeal,” a sheriff’s spokesman said last week.

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The Sheriff’s Office resubmitted the case in late 2015, after three other jail guards beat a mentally ill inmate to death, and District Attorney Jeff Rosen’s office then filed charges last July.

Another factor that may affect the retrial decision is that a female juror last week during closing arguments felt so strongly about Tri’s innocence that she loudly exclaimed “yeah,” in support of an objection made by Tri’s defense attorney during the prosecution’s rebuttal. Despite the woman’s outburst, Judge Paul R. Bernal declined to replace her with an alternate juror after the woman insisted she could still be fair during deliberations.

Tri was fired by Sheriff Laurie Smith after the 2013 incident, which left inmate Danny Jackson with fractured sinus bones and two facial cuts on his face that required stitches.

During the trial, prosecutor John Chase contended that Tri used unreasonable force by allegedly kicking or stomping Jackson in the head and face. At the time, Jackson was either straddled or being held down by two officers, including correctional deputy Kala Bragg, who is 6-feet-5-inches tall and outweighed Jackson by about 95 pounds.

Bragg, a witness for the defense, testified that he did not see Tri kick Jackson. But under cross-examination by Chase, he acknowledged that he wrote a report saying that Tri had kicked the inmate in the head he and Tri talked about what had happened.

Officers are trained to avoid hitting an inmate in the head unless they are under serious threat.

Defense attorney Erin M. Dervin contended that Tri acted in defense of himself and the other officers after Bragg pressed an emergency button summoning Tri and other officers to the scene.

Bragg said he pressed the button even though Jackson hadn’t attacked him physically because he felt threatened when the inmate refused to back off after getting physically “in my face.”

Tri’s trial was held in Morgan Hill rather than San Jose, where the Oct. 3, 2013 incident took place, because a judge was available in the South County courthouse.

The case was part of an unprecedented wave of prosecution in the two years since three officers beat mentally ill inmate Michael Tyree to death. Altogether, District Attorney Jeff Rosen’s office has charged six jail guards in three excessive force cases, an unrivaled number spurred by Tyree’s fatal beating in August 2015.

Last month, a jury convicted the three jail guards of Tyree’s murder, and they now face 15 years to life in prison.

In a separate case, two other guards now face assault charges in the beating of a shackled inmate about a month before Tyree died. That case, against correctional deputies Phillip Abecendario and Tuan Le, could go to trial late this year or early next year.