A weapon in the hands of someone with a history of violence or mental instability is dangerous for anyone, but particularly for victims of domestic violence.

More than 810,000 people were victims of violence from an intimate partner in 2012, and the majority of victims (84 percent) were women. More than half of intimate partner murders involve a firearm. Those who abuse their partners tend to offend again, and they’re far more likely to kill their victims if they own a firearm.

“To state it in plain English, you can’t commit a crime with a gun if you don’t have a gun to do it with,” said Garen Wintemute, director of the UC Davis Violence Prevention Research Program.

Federal and state statues prohibit specific groups of people from owning a firearm: those who have been convicted of a felony, those who have had a mental breakdown (to the point of hospitalization), or those who have been convicted of a domestic violence crime.

In California, those prohibited persons must surrender their firearms immediately if a law enforcement officer makes a demand for them. Beyond preventing offenders from purchasing firearms from licensed retailers, however, the law has been challenging to enforce.

“The laws have been on the books for a while that you have to do it, but there was never any teeth,” said Sgt. Lynda Gibbons, who oversees the Major Crimes Unit and the Domestic Violence Compliance Unit in San Mateo County.

In recent years, California has developed a database of barred gun owners, the Armed Prohibited Persons System (APPS), and now several initiatives are using this list as a key resource to keep weapons out of the wrong hands. Currently 21,202 people are in the APPS system, and there are 43,842 guns associated with those names.

The APPS list was completed in 2006, and law enforcement conducted the first sweep in 2007, according to Nick Pacilio, a DOJ spokesperson. California is currently the only state that has a system like this in place: one that both maintains a list of those who have purchased a firearm, cross-references that with a list of those who are prohibited from owning firearms, and then has law enforcement officials follow up with those individuals, Pacilio said.

Attorney General Kamala Harris added 10 agents to the program in 2011, bringing the total number to 33 agents who were assigned to track down the nearly 20,000 people named on the APPs list. In May 2013, Governor Jerry Brown signed Senate Bill 140, which granted an additional $24 million to add additional investigators.

They’re likely to need the extra help: in 2014, the list will expand to include the transfer of rifle and shotguns, something not previously tracked, which will likely expand the number of people who need to be tracked down.

To enforce the APPS list, investigators track down the prohibited person, knock on doors, and ask for the guns back. If they’re told the weapon is not there or it’s been sold, the officers will ask to search the house. If they are denied, and the investigators deem it necessary, they will get a warrant to search the house.

In many cases, Wintemure said, simply asking for the weapons back has been sufficient. “Most of the time they don’t need a warrant.” Very often, he added, the prohibited person would be willing to allow law enforcement into the home to collect any firearms, because it is a far more serious offense to have them as a prohibited person.

Some counties, such as Santa Cruz County, learned about the program and have opted to take the matter into their own hands rather than relying on state investigators.

“Santa Cruz is a mid-size county and probably not at the top of the priority list,” said Santa Cruz County District Attorney Bob Lee. “We didn’t want to wait,” he said.

Lee requested the names of prohibited persons living within the county. He assigned his own investigation team – currently one attorney, two investigators, and a staff person – to check out the names on the list, but they soon discovered it was a more complicated process than he originally thought.

Much of the information on this list – which was created in 2006 – is outdated, Lee found. People may be listed under the wrong address, they may have moved out of state, and others have had their rights reinstated or their charges dismissed.

It wasn’t long before the team realized they would be more effective if they could get search warrants to go inside the homes of these people – simply knocking on doors wasn’t enough.

Beginning with those persons with all three restrictions – domestic violence offenders, felons, and those with mental health problems – they began collecting information to establish criteria for search warrants, which Lee says requires more legwork than a neighborhood sweep.

“It’s an undertaking, but we’re glad to do it,” Lee said. “We think it’s more effective than a buy-back program, by taking guns from those citizens who shouldn’t have them in the first place.”

San Mateo and Butte counties have also been home to a similar program to collect weapons. However, the focus in the program in these counties was to specifically target domestic violence offenders. The initiative took place between 2007 and 2010, and Garen Wintemute and his team of researchers conducted an analysis of the programs and determined that their methods were effective in taking guns away from abusers.

The process in these counties went like this: when a restraining order against a domestic violence perpetrator was issued, that information was relayed to the detectives on the program. They would do what the larger APPs program does – look at computerized data that included handgun transfers, assault weapon registration, permit applications, etc. but they recognize that not all gun owners operate within the law.

“Quite frankly there are people flying under the radar that may have a gun, but it’s not registered to them,” Gibbons said. She explained that the registered gun owners – those in the APPs database – were getting caught, while others were skirting the authorities.

She explained that it’s not uncommon for convicted felons to have a firearm that’s not registered to them – they’re either going to buy a stolen gun or somehow keep it under the radar.

The program investigators would examine court documents and the restraining order petition filed – items not included in the state database – and sometimes they would speak with the petitioners (the victims) or their attorneys to gather information on whether the abuser had a gun.

“That’s when we rely on the protected parties reporting of the information and as to, yes this person does have a gun, this is what it looks like, this is where he keeps it…and if we can verify and corroborate that information, we’ve been successful in obtaining search warden warrants to go and recover those firearms,” Gibbons said.

The program was only funded for a few years; however, San Mateo County secured federal funding that enables their program to continue. Gibbons said this focus on domestic violence offenders is important work, in addition to the efforts of the state investigators using APPs.

She pointed to research indicating that women are more than twice as likely to murdered by an intimate partner with a weapon, than by a stranger with any other weapon.

“Just by the sheer load of the amount of domestic violence restraining orders issued in every county, it would tsunami the DOJ agents,” she said. “They just wouldn’t be able to keep up with every restraining order that’s issued.”

Gibbons said the San Mateo program has seized over 1,000 firearms since its inception in 2007.