TUCSON, Ariz. — Jared Loughner had trouble with the law, was rejected by the Army after flunking a drug test and was considered so mentally unstable that he was banned from his college campus, where officials considered him a threat to other students and faculty.

But the 22-year-old had no trouble buying the Glock semiautomatic pistol that authorities say he used in the Tucson rampage Saturday that left six dead and 14 injured, including Rep. Gabrielle Giffords.

Loughner’s personal history did not disqualify him under federal rules, and Arizona doesn’t regulate gun sales. His criminal charges were ultimately dismissed, the Army information was private and Pima Community College isn’t saying whether it shared its concerns about Loughner with anyone besides his parents.

Loughner cleared a federal background check and bought the pistol at a big-box sports store near his home on Nov. 30 – two months after he was suspended by the college. He customized the weapon with an extended ammunition clip that would have been illegal six years earlier.

There is nothing to indicate that anything went wrong in the process leading up to that purchase – except the ultimate outcome. But the question hangs: Was there any single piece of behavior – or a combination of two or more – that might have prevented Loughner from obtaining the gun that police say he used during his rampage?

Background checks are designed in part to weed out prospective gun buyers who have felony criminal records, have a history of domestic violence or are in the country illegally. None of that applied to Loughner.

There were warning signs, but nothing in his past that should have disqualified him under the laws and regulations as they are written today.

Gun-control advocates say the shooting shows that Arizona, home of some of the nation’s most permissive gun laws, must review its laws to make sure firearms are not falling into the wrong hands. Gun-rights proponents disagree and say more regulation would not have stopped the tragedy.

Arizona eased gun restrictions last year when it passed a law allowing residents 21 and older to conceal and carry a weapon without a permit, which allowed Loughner to furtively – and legally – carry his pistol to the mall where he is accused of opening fire.

No permits or licenses are required at the state level. Gun owners can bring concealed weapons into Arizona bars and restaurants, and state legislators are considering allowing students and teachers to have weapons in schools.

After the shooting, Pima County Sheriff Clarence Dupnik berated Republican lawmakers who have sought to further ease state gun laws.

“I think we’re the Tombstone of the United States of America,” the Democrat said, referring to the Wild West town populated by gunslingers. “I have never been a proponent of letting everybody in this state carry weapons under any circumstances that they want, and that’s almost where we are.”

Charles Heller, co-founder and secretary of an Arizona group that promotes gun rights, said more regulation is not a solution.

“Why don’t we ban murder? . . . Murders are illegal and people do it anyway,” he said. “There is no way to weed people out.”

Outside Sportsman’s Warehouse, the cavernous store where Loughner purchased his Glock, gun owner Jason Moats said that “the bad guys can get the guns either way.” He suggested that the shootings could have been less tragic had there been one more weapon out there, rather than one less.

If someone at the mall was armed and had shot Loughner, ending the attack, “the guy would be a hero,” said Moats, a 25-year-old route manager for a waste hauling company.

Federal law bars gun ownership for people who’ve been judged dangerously mentally ill by a court and those who have been committed to a mental institution, thresholds that didn’t disqualify Loughner. Less than 1 percent of the federal government’s background-check rejections involved mental-health issues, according to FBI records.

“It’s not easy to draw that line” of when a person’s mental illness should disqualify them from owning a weapon, said Michael J. Fitzpatrick, executive director of the National Alliance on Mental Illness, an advocacy group.

“The reality is most people with mental illness are not violent,” he said. “The issue, frankly, is getting people into treatment. It’s not about guns.”

Daniel Vice, a senior attorney with the Brady Center to Prevent Gun Violence, said something should have prevented Loughner from buying a gun so easily.

“Here is a guy who couldn’t enlist in the military and was kicked out of school. Anyone would tell you don’t give this guy a gun,” Vice said.

He said Loughner’s problems might have been detected in states with more restrictive permits, including New Jersey, Massachusetts, Hawaii and Illinois.