The gunman accused of trying to assassinate Arizona Rep. Gabrielle Giffords and killing six others, Jared Lee Loughner, was not on any government watch list that might have warned someone not to sell him a gun or caused police to investigate his unstable behavior.

It turns out there is not a list in the United States for people like Loughner.

The same goes for Joseph Stack, who flew his plane into an IRS office in Austin, Texas, last February. Stack left behind a 3,000-word, rambling screed about his problems with the U.S. tax code.

Less than a month later, John Patrick Bedell shot two Pentagon guards. He left behind anti-government writings and cited conspiracy theories involving the U.S. military.

Richard Poplawski, too, left an online trail of racist rants and paranoid thoughts about President Barack Obama imposing a gun ban before he allegedly shot and killed three police officers in the Pittsburgh area in April 2009.

In the past two years, there have been at least six incidents in which disgruntled Americans, acting alone, have taken violent action into their own hands. In many of the cases, signs of government distrust and paranoia wouldn't have been enough to justify law enforcement intervention.

Loughner's case includes cryptic messages left on a MySpace page, bizarre behavior in college classes and YouTube videos with anti-government rhetoric. Yet it wasn't enough to put him on the radar screen of authorities as a possible violent person.

"This is a very difficult individual to find, to detect, minus any kind of mental evaluation or criminal violence he committed or suspicious activity that somebody reported that was in the system," said Mike Downing, deputy chief of the Los Angeles Police Department who oversees counterterrorism and special operations.

The six who died were among 19 people shot Jan. 8 outside a Tucson grocery store, where Giffords was meeting with constituents. Investigators were looking into whether the 22-year-old Loughner was part of an online anti-government organization, American Renaissance. But participation in such groups and likeminded beliefs are not crimes.

"Law enforcement can't just jump in there and monitor all these groups just based on the message they're sending," said Don Borelli, a former assistant special agent in charge of the FBI's New York joint terrorism task force.

There are scores of domestic groups with members who oppose paying taxes, disagree with the government and voice their opinions eagerly. But their rights are protected by the First Amendment, and opposing taxes alone is not enough to trigger an investigation.

"These guys kind of fly below the radar until they decide to act, which makes it a challenge for law enforcement," said Borelli, who is now a senior vice president with the Soufan Group, an international firm that consults on security issues.

Neighbors described Loughner as someone who kept to himself and walked his dog. Some of his friends said they were surprised when he expressed an interest in guns and target shooting last March.

Between February and September, Loughner had five contacts with Pima Community College police for classroom and library disruptions.

School officials are responsible for contacting police if they believe a student is a threat to himself or others, said George Foresman, a former undersecretary at the Homeland Security Department. If a friend or teacher did call the police department to report that Loughner was posting rambling messages about illiteracy rates and currency online, it's not likely the local police would have a system in place to run that down.

"They're dealing with a series of open cases, murders, robberies," Foresman said. "Where does this fall into the priority?"

Loughner was suspended in September after the college police found a YouTube video in which Loughner claimed the college was illegal according to the U.S. Constitution. But security experts said it was hardly enough to raise suspicions.

"Students get kicked out of school for a multitude of reasons," Borelli said. "That doesn't necessarily mean that they're going to go kill somebody."