The California Department of Justice has maintained the Armed Prohibited Persons list since 2007. It is supposed to keep guns from the 18,000 people listed who purchased guns legally but are now not allowed to own one.
"In some instances it's as simple as knocking on the door, in some instances they could be on parole or probation, we contact their parole or probation officer and we just kind of work it through that process and it's been very successful for us for these first couple months that we used it," San Francisco Police Chief Jeff Godown said.
The list matches registered gun owners with court records of people who have been convicted of felonies and certain misdemeanors, have a restraining order, are mentally ill or otherwise are prohibited from owning a gun.
San Francisco police began using the list two months ago. They had over 200 people on their list and have already looked into half, seizing 53 guns.
"Every gun I get off the streets is one less gun that has the potential of being used in a crime in this city," Godown said.
Hundreds of Bay Area residents are in APPS. Law enforcement agencies get yearly reminder notices to sign up for APPS and each month the state Department of Justice provides an updated list to police departments who ask for it.
But many police departments do not subscribe or make an effort to follow up on the list.
The state Bureau of Firearms says 37 police departments and three county sheriff's offices in the state still have not done so.
Roy Perez is one reason the list exists. Perez was on the list because he displayed erratic and violent behavior in the past and had been placed on a 72-hour psychiatric hold more than once. But police never took his gun away.
In 2008, Perez shot his mother 16 times in her Baldwin Park home in Southern California. He then went next door and killed a neighbor and her 4-year-old daughter. Perez pleaded guilty and will spend the rest of his life in prison.
"There needs to be a stronger process for getting people to relinquish their guns at the time that they become prohibited," Legal Community Against Violence spokesperson Ben Van Houten said. "There have been significant studies done that show a significant connect between people with past misdemeanor convictions and arrest and the likelihood of future crimes committed."
But there are problems with APPS. In many cases, the registered address of the gun is not always where it is today. And the list does not match individuals who purchased long guns. In California last year, more long guns were sold than handguns.
Special agent John Marsh is with the state Bureau of Firearms. His department generates the APPS list.
"The Bureau of Firearms investigates the most people in the APPS system," Marsh said.
Each year, the bureau investigates more than 1,700 people and seizes more than 1,200 firearms.
"Anywhere from hundreds of guns from a single individual to a single gun; but a gun is a gun and it can be used in a, you know, in a harmful way," Marsh said.
Marsh is one of a handful of state officers who confiscate guns.
"The bureau has 20 agents, and that's from the Oregon border to the, you know, to San Diego," Marsh said.
Those officers also work many other cases. That means for the list to be effective and local police and sheriff deputies have to help.
We provide them with a list of people in their city that are prohibited from owning firearms and then if they have the man power available then they can investigate those people," Marsh said.
The bureau says many police departments facing budget cuts cannot afford to follow up and there is no uniform policy on what each department does with the list.
Oakland police have used apps for more than two years. There are a lot of names on their list.
"Last time I checked it was in the 400 range for Oakland," Oakland Police Officer Kevin Kaney said.
An undercover officer gets the information and then gives it to patrol officers to follow up.
"We found firearms with drugs we've found firearms with drugs and large amounts of cash," Kaney said.
San Jose police just use the list as a reference tool, but do not actively pursue prohibited gun owners.
Nearby Menlo Park does not subscribe.
East Palo Alto and Redwood City police departments had not subscribed to the database but would now that ABC7 asked them about it.
"I tell other agencies if you are not using that list it's a really a great resource it's worked out well for us," Godown said.
Police say many people just do not know what to do with the guns when they are told they are not allowed to have them. San Francisco police gave an example of one woman who buried her son's gun in the backyard. They suggest people call their local police and arrange for a hand-off
This story was the result of findings of a joint investigation by ABC7 and freelance reporter Ed Connolly into the Armed Prohibited Persons (APPS) program.
Written and produced by Ken Miguel