When Jared Loughner went to the Sportsman's Warehouse near his home Nov. 30, there was nothing to stop him from buying the gun authorities say he used in Saturday's shooting rampage.

He didn't fit any of the categories of "prohibited possessors" defined in federal or state law. He passed an instant federal background check and was on his way with a Glock semiautomatic pistol that sells for around $550.

Pima Community College officials had suspended Loughner from the school and demanded in October that he receive a mental evaluation showing that he was not a danger to himself or others before he could return to school. But that was not near the threshold required to prohibit someone from buying a gun.

Federal law establishes two categories of people who can be prohibited from buying a gun because of their mental incompetence, said Tucsonan Charles Heller, co-founder and secretary of the Arizona Citizens Defense League, a pro-gun-rights group. They are:

• Those who have been incarcerated in a mental health facility against their will.

• Those who have been accused of a crime and found mentally incompetent to stand trial.

Otherwise, in Arizona, there is little to stop even an adult who seems mentally unstable from buying a gun.

The Bureau of Alcohol, Tobacco, Firearms and Explosives said Loughner's purchase was corroborated by store receipts and video, among other evidence, according to a search warrant return released Sunday.

To Heller, Saturday's shooting rampage, which killed six and injured 14 outside a northwest-side Safeway store, is evidence of the wisdom of liberal gun laws.

"This shows why it is so vital to have an armed citizenry," Heller said. "If you can't get the guns out of society, what can you do? You can have a well-prepared citizenry."

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But Pima County Sheriff Clarence Dupnik derives a contrary conclusion from Saturday's shooting.

"I have never been a proponent of letting everybody in this state carry weapons under any circumstances that they want," Dupnik said. "And that's almost where we are."

In a tragic coincidence, the gun used in Saturday's shooting is the same type of gun that Giffords said in a 2008 election campaign that she owned.

The Washington Post contributed to this report. Contact reporter Tim Steller at 807-8427 or at tsteller@azstarnet.com