The federal Bureau of Alcohol, Tobacco, Firearms and Explosives is trying to determine who owned the gun and how it was obtained.
"It sounds like the problem is that folks knew he was dangerous but our weak gun laws allowed him to be armed," said Daniel Vice, an attorney for the Brady Center to Prevent Gun Violence, a national lobbying group.
He said the federal law is designed to prevent someone who is involuntarily committed to a mental institution from owning a gun.
"It was illegal for him to possess a gun but there is a loophole that, if you are buying a gun from a private seller, there is no background check required. It is still illegal to buy the gun, but there is no way the seller would know that," Vice said. "These preventable shootings happen over and over."
When Helen Jansen called for help on Friday, she said her son was distraught and that he could not be subdued. Paramedics sought backup from the three deputies as they tried to treat Jansen. Investigators said the deputies had a lengthy conversation with Jansen before he pulled out a gun.
Members of Jansen's family said Tuesday they didn't want to talk about the case.
The court records show Helen Jansen was increasingly worried about her son in the petitions for his involuntary commitment filed from December of 2009 to April 2010.
In the first petition on Dec. 15, 2009, Helen Jansen said that her son would not take his medications, was "extremely depressed," told people he was a Marine Corps general who had served in Afghanistan and had been arrested for making obscene gestures to passing motorists. The petition described him as: "A real and present threat" to himself and to others. A judge quickly approved the petition, but it was dismissed on Jan. 12, 2010, after a psychiatrist wrote that he was taking his medications and could be treated on an outpatient basis.