Under federal and Arizona law, a mentally ill person is barred from purchasing a gun if a court has deemed that person a danger. And in hindsight, there is evidence of Mr. Loughner's disturbed mental state.

At Pima Community College, Mr. Loughner had five contacts with campus police. The school suspended him last year and said he couldn't return unless he obtained clearance from a mental-health professional that indicated his presence wouldn't pose a danger to himself or others.

But a federal law-enforcement official on Sunday said there was no evidence Mr. Loughner had been under any court-ordered treatment, which is one clear route to barring someone from buying a gun. Officials said he bought his gun legally Nov. 30 at a Tucson outdoor-sports store after a full background check.

"You can be diagnosed paranoid schizophrenic and buy a gun," said an official with the Bureau of Alcohol, Tobacco, Firearms and Explosives. "A judge has to find you mentally ill before you are prohibited."

The shooting sparked debate Sunday on gun laws' adequacy. Arizona's are among the least restrictive in the U.S., but Republican lawmakers argued that no new laws would have stopped Mr. Loughner. Democrats countered that it is at least legitimate to examine how Mr. Loughner obtained a weapon.