In California, the laws are strict, but thousands of people have weapons even though they're banned because of mental illness or violent histories.
The department of justice is trying to change that, and Action News Reporter Corin Hoggard was there as they raided Valley neighborhoods.
Jared Loughner is the 22-year-old man suspected of shooting Congresswoman Gabrielle Giffords and killing six other people outside a Tucson, Arizona, grocery store this January.
Loughner had been kicked out of community college for an apparent mental illness.
Federal law bans people who have been committed to a mental institution or deemed mentally ill by a judge from owning firearms. But Loughner's school never reported the problems he had on campus, so there was nothing to stop him from buying the gun police say he used.
California's department of justice is cracking down on the people they can stop.
Agents say, in five counties of the Central Valley, almost 12-hundred people who are banned from owning guns, because of mental illness or previous violence, own guns anyway.
John Marsh: "I think they're a risk to the community. I mean, somebody's deemed them to be a little unstable."
Agents say many unstable people operate in a gray area. They hold onto weapons they had before they were banned from owning firearms, or buy them from shady dealers, often in states with more lax gun control laws.
The department of justice goes after some of them every year. But less than two months after the Tucson tragedy, Action News learned agents launched an eight-week sweep, knocking on doors up and down the state.
In Clovis, they're serving a warrant to get the weapons from this house right here. They say the man inside is mentally unstable. In fact, he even built a tripwire in front of his door so he would know if police came to his house.
The sign at Robert Norris' home says "keep out" and neighbors told me he sometimes referred to his bunker-filled yard as his "kill zone."
Armed with the warrant, a battering ram, and several high-powered weapons of their own, officers got inside the house.
Minutes later, they carried weapons out of the house in bunches. In all, they found two handguns, three shotguns, and two rifles, plus 500 rounds of ammunition.
Norris told neighbors he suffers from post traumatic stress disorder after a tour of duty in Vietnam. They say the sight of police cars at his house wasn't much of a surprise.
"He's always been a little different. You know, he's been a little strange."
This neighbor didn't want us to show his face because he felt threatened by Norris after an incident just three days before the department of justice showed up.
"We got about 100 feet away from his yard and he comes darting out, pulls a knife on us and starts telling my buddy he's gonna fix his mouth for him."
Agents arrested Norris on felony charges for owning weapons in violation of a weapons ban handed down just a month earlier. Agents say he's now getting medical help from the Veterans Hospital.
During the same sweep, agents also targeted people prohibited from owning firearms because of issues with domestic violence.
In Northwest Fresno, they arrested John Thomas Sullivan and took three weapons from his parents' home, including an unregistered AK-47 assault rifle. Sullivan has a domestic violence conviction from 2001 and agents say he could still pose a danger.
John Marsh: "He knew he shouldn't have guns and then having an assault weapon is definitely notching it up."
Sullivan's father told me his son thought the gun ban against him had expired. The district attorney's office is now considering whether to file charges.
The sweep ended last week and agents aren't saying yet how many people they arrested or how many weapons they seized. But to make an impact, they say one arrest or one seized weapon could be enough.
John Marsh: "If we take guns from one person, we don't know if that person has the intent to do something illegal with the gun."
By shining a light on potential problems, these agents hope they've prevented any future tragedies like what happened in Tucson.