SAN FRANCISCO (CBS SF/AP) — A San Francisco police officer was found not guilty in the 2019 beating of a man with a baton in what's believed to be the city's first trial against an officer over excessive force allegations while on duty.

A jury on Monday found Terrance Stangel not guilty of three charges of assault and battery he faced after striking Dacari Spiers with his baton several times, breaking his wrist and leg.

The jury deadlocked on a fourth charge of unlawfully beating Spiers under color of authority.

Stangel and his partner, Officer Cuauhtemoc Martinez, were responding to 911 calls about a man choking and dragging a woman near Fisherman's Wharf when they encountered Spiers and his then-girlfriend on Oct. 7, 2019.

Martinez, who was not charged in the case, immediately grabbed Spiers while ordering him to get against the wall and neither Stangle nor Martinez gave him reasonable commands before Stangel started beating him with his baton, landing at least seven blows, Young said.

"Five strikes when he's lying on the ground, in the fetal position, writhing in pain, is proof beyond a reasonable doubt of assault and battery," Young said during closing arguments in the three-week trial.

Stangel was charged with four felonies, including battery causing serious bodily injury, assault with a deadly weapon, assault with force likely to cause great bodily injury and assault under the color of authority.

Stangel testified that his aim was to protect his partner from a violent man after the interaction between Martinez and Spiers quickly turned into a melee.

"I was trying to get him to stop fighting my partner and I was trying to us get out of the situation without getting hurt," Stangel said.

His defense attorney, Nicole Pifari, said Stangel used necessary force to control a violent situation created by Spiers and said the case was politically motivated.

"Their case is terrible. They had to create an alternate reality to bring this case," Pifari said told the jury during closing arguments. "They lied to you."

Stangel is one of six officers that have been charged by San Francisco District Attorney Chesa Boudin, a former public defender who was elected DA in 2019 as part of a national wave of progressive prosecutors opposed to mass incarceration.

Following the verdict, Boudin issued a statement which read in part, "We respect the jury process, although we remain disappointed that police accountability remains so elusive and difficult to achieve. I am committed to continuing to hold those who commit harm accountable—regardless of the uniform they may wear or the badge they may carry.  No one should be above the law, and my office will continue to fight to ensure that all communities are safe."

The prosecution became a flashpoint between Boudin and Police Chief Bill Scott amid competing claims that both agencies had withheld evidence.

A district attorney's office investigator testified she felt pressured to sign an affidavit against Stangel that left out evidence that could possibly have helped him but U.S. District Court Judge Jacqueline Scott Corley ruled that the evidence would not have affected the case. She also fined the city, saying that the police department failed to disclose three interviews with officers who were involved, Mission Local first reported.

During the trial, Scott terminated an agreement that allows the district attorney's office to investigate police shootings excessive use of force and in-custody deaths, citing serious concerns over the office's impartiality. Both agreed to let the agreement stand for another two months after the state attorney general's office intervened.

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