Under new law, officers could face criminal charges for using lethal force when there was a reasonable alternative

California has adopted the strictest law in the United States limiting when police can use deadly force.

Come 1 January, law enforcement in the state will only be allowed to do so if the “officer reasonably believes ... that deadly force is necessary to defend against an imminent threat of death or serious bodily injury”.

Gavin Newsom, the California governor, signed the legislation on Monday, surrounded by families holding up photos of loved ones killed by police throughout the state.

“As California goes, so does the rest of the United States of America,” Newsom said. “We are doing something today that stretches the boundaries of possibility.”

Newsom signed the bill into law the same day the New York police department fired an officer involved in the death of Eric Garner, an unarmed African American man killed by police on Staten Island in 2014.

The legislation was inspired by the 2018 police shooting of Stephon Clark, an unarmed black father of two killed by police in his grandparents’ backyard in Sacramento when officers mistook his cellphone for a gun.

Under this new law, should investigators determine that an officer used lethal force when there was a reasonable alternative, the officer could face criminal charges, civil liability and disciplinary action.

“California should and will ask more of their officers,” said Peter Bibring, the director of police practices at the American Civil Liberties Union of Southern California. The ACLU was one of the sponsors of the bill.

“This is landmark legislation that makes California a model for other states,” Bibring continued. “We’ve seen this standard put into practice with other police departments. We expect this legislation will save lives.”

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The current constitutional case law standard across the US sets the bar for use of force at “objectively reasonable”, which civil rights advocates and families of people killed by police have long criticized as being too low. The current law allows officers to argue a shooting was justified if they feared for their lives and acted in self-defense. Prosecutors have used it to justify not bringing criminal charges against officers in the shootings of unarmed civilians.

California police kill people at a rate 37% higher than the national average per capita, according to the ACLU. Police in the state killed 172 people in 2017, more than two-thirds of whom were people of color. Police in Kern county, California, killed more people per capita than in any other American county in 2015, a Guardian investigation found.

“Willie was about a month shy of his 21st birthday when he was killed,” said Kori McCoy, brother of Willie McCoy, a rapper who had been sleeping in his car at a Taco Bell when Vallejo police fired 55 times at him. “It saddens me that none of us will ever get to see what kind of man Willie would have been at 30, or 40, or 50. We do, however, have the opportunity to see what kind of state California will become.”

The bill faced serious opposition from law enforcement unions and had to undergo several amendments before it passed in the state assembly and the senate. The most serious of amendments included removing the requirement of de-escalation.

Barack Obama’s task force on 21st century policing had recommended that police officers create time and distance and de-escalate tensions when interacting with subjects, and the concept has been adopted as the preferred course of action throughout law enforcement. But law enforcement unions opposed codifying it in the bill, as officers would have been found criminally negligent and civilly liable for not taking steps to de-escalate before shooting.

“We knew that it would be an uphill battle, especially with police associations opposing the bill,” said Melina Abdullah, the Black Lives Matter Los Angeles chapter lead. “Unfortunately, in efforts to get law enforcement to lift their opposition, the bill was so significantly amended that it is no longer the kind of meaningful legislation we can support.”

The state assemblymember Shirley Weber, who co-authored the bill, acknowledged that it had been “a hard and a long journey” but said the new law “changes the culture of policing in California”.