LOS ANGELES — After a mass killing in Santa Barbara in 2014, California passed a law that let police officers and family members seek restraining orders to seize guns from troubled people. A year later, a shooting rampage in San Bernardino led to voters approving a ballot proposition to outlaw expanded magazines for guns and require background checks for buying ammunition.
The state has also banned assault weapons and regulates ammunition sales — all part of a wave of gun regulation that began a quarter century ago with a mass murder at a San Francisco law firm.
California may have the toughest gun control laws in the nation, but that still did not prevent the latest mass killing — a shooting on Wednesday that left 12 people dead at the Borderline Bar & Grill in Thousand Oaks.
The community of Thousand Oaks is just starting to grieve its losses, and investigators are still combing through the background of the gunman, who was found dead after the shooting. But gun control activists and politicians in the state are already weighing what more can be done, and whether existing measures could have prevented the killing.
The attack came just as California elected a new governor, Gavin Newsom, this week, and eyes are on him to see how he responds.
Mr. Newsom is seen as even more aggressive on gun restrictions than his predecessor, Gov. Jerry Brown, and some experts say the state could see the passage of even tougher laws. As lieutenant governor, Mr. Newsom led the effort after the San Bernardino killings to pass the ballot proposition on high-capacity magazines and background checks — a measure that has not been enacted yet because of a court challenge.
With Mr. Brown out of the state this week, and Mr. Newsom serving as acting governor, the mass shooting became the first crisis he faced after being elected Tuesday night.
“The response is not just prayers,” Mr. Newsom said at a news conference on Thursday in Sacramento. “The response cannot just be excuses. The response sure as hell cannot be more guns.”
Survivors and family members of those who have been killed in gun violence are also calling for stronger measures. On Thursday, Susan Orfanos, whose son survived a mass shooting in Las Vegas last year only to die in the Borderline, told a New York Times reporter: “He didn’t come home last night, and the two words I want you to write are: gun control. Right now — so that no one else goes through this. Can you do that? Can you do that for me? Gun control.”
As California has become more liberal in recent decades, and especially after President Trump was elected, gun control is one of several issues — along with climate change, immigration and health care — that have placed the state firmly in opposition to the federal government. In the wake of mass killings, the state’s political leaders often find themselves pushing for more gun control within California while speaking out against the federal government’s unwillingness to take up the issue, and against the National Rifle Association’s positions.
“The National Rifle Association — I’ll say this — is bankrupt, morally, and they need to be held to account to their rhetoric and their actions,” Mr. Newsom said.
He said of his own state: “California will do its job. I have all the confidence in the world. We have an extraordinary Legislature; they get it. They’ve gotten a lot done on gun safety, and they’ve got a governor who wants to raise the bar.”
Kevin de León, the former Democratic State Senate leader who on Tuesday lost his bid for the United States Senate to the incumbent, Dianne Feinstein, wrote on Twitter this week: “Congress’s great shame is their willingness to give the N.R.A. an outsized voice on how we enact gun safety. We will keep marching, legislating, advocating, and pushing Congress until they finally deliver gun safety laws that will keep our communities protected.”
Mr. Newsom did not this week offer specific new measures that he would push for, but he did say that he would have signed some gun control bills that Gov. Jerry Brown had vetoed in recent years. Among those were bills that would have expanded restraining orders, to allow co-workers, school employees and mental health providers to ask courts to take away guns from someone.
Even with the country’s toughest gun laws, California has still had the most deaths from mass shootings since 1982, according to a database compiled by Mother Jones — 128 people killed. Florida, with roughly half the population of California, has the second most deaths from mass shootings over that time, 118 killed.
But California also has the highest population in the country, and no one knows how many mass shootings may have been prevented by the gun laws already in place in the state.
“What matters is not just the count but the rate,” said Garen Wintemute, an emergency room physician who also leads the University of California Firearm Violence Research Center, which was created in the aftermath of the San Bernardino massacre to study how to prevent mass shootings. “And California’s rate is about half that of Florida’s.”
Most gun deaths are not from mass shootings, and the focus of the gun control movement is on reducing the overall number of gun deaths — in homicides, suicides and accidents. By that measure, California has been successful: It has cut its gun-death rate in half over the last 25 years, and California is among the states with the lowest rates, with 7.9 deaths per 100,000 residents in 2016, according to data from the Centers for Disease Control and Prevention.
Despite its reputation as tough on guns, there are pockets of support for easing controls, especially in rural, inland areas, and there are still plenty of gun owners in the state. California has the second most registered guns in the country — more than 340,000, which is second to Texas, according to statistics published by the Bureau of Alcohol, Tobacco, Firearms and Explosives. But on a per-capita basis, California ranks low, 44th, with 8.71 guns per 1,000 people.
Ian D. Long, the gunman in the Thousand Oaks shooting, and a Marine who had served in Afghanistan, drew the attention of police officers in April when they were called to his house for a domestic disturbance. Mental health specialists were called in, and discussed with him his military service and possibility of post-traumatic stress disorder, but determined he was not dangerous enough to detain him and force him to receive treatment.
The episode though raised the question of whether someone should have tried to keep him away from guns by seeking a restraining order. “My issue with Thousand Oaks is implementing the gun violence restraining order we already have in place,” said Allison Anderman, managing attorney at the Giffords Law Center to Prevent Gun Violence, an advocacy group in San Francisco. “The shooter was undeniably a candidate for a gun violence restraining order and for whatever reason the Ventura County Sheriff’s Department did not try to obtain one.”
The predecessor of Giffords, the Legal Community Against Gun Violence, was founded in the wake of another mass shooting in California, the so-called 101 California Street Shooting in 1993, when a man armed with semiautomatic guns killed eight people at a law firm in San Francisco. That shooting, long before the massacres in Columbine, Colo.; Newtown, Conn.; or Parkland, Fla., prompted a flurry of gun control activism in California, and played a large part in the enactment of the federal assault weapons ban in 1994, which is no longer in effect.
Mr. Long also apparently used an extended magazine on his Glock 21 .45-caliber handgun, a device that California voters banned with the approval in 2016 of Proposition 63. Mr. Newsom, who led the effort, called the initiative “historic progress to reduce gun violence.” But the measure has stalled in the courts after a lawsuit from the N.R.A.
In the aftermath of the Thousand Oaks shooting, activists are pushing to expand background checks and widen the circle of people who can seek gun restraining orders. Dr. Wintemute, the emergency room physician, said he was pushing to make a history of alcohol abuse a criterion to prohibit someone from buying a gun. “I think we’ll see interest in tightening up California’s background check procedures,” he said.
In a column about the shooting published in The Sacramento Bee, Dr. Wintemute wrote: “Unfortunately, many background checks are inadequate. Information is not reported or is mishandled; legal ambiguities can make it difficult to know whether events are, in fact, prohibiting. And even where background checks are required, many people avoid them.”
In 2014, Bob Weiss’s daughter, Veronika, was killed in the shooting in Santa Barbara County, in the community of Isla Vista. His home is walking distance from the Borderline bar in Thousand Oaks.
“I think for most of the country, it’s routine now,” he said. “People die in Pittsburgh, a week or later 12 die here, next week 12 people get shot somewhere else. We’ve been desensitized. Even though it’s getting worse, we’ve all just come to accept it. I don’t accept it.”
Mr. Weiss attended a vigil in Thousand Oaks on Thursday night.
He said he favored stricter gun laws, and saw parallels between the shooting in Thousand Oaks and the one that took his daughter’s life just a few years ago.
“Just like what happened in Isla Vista, they had contact with this person and they deemed him to be no risk to the public,” he said. “They didn’t even ask if he had a gun, which is the most basic question you should ask someone who is displaying these violent tendencies.”
An earlier version of this article misstated the rate of gun deaths in California in 2016. The state had 7.9 deaths per 100,000 residents, not per 1,000 residents.