For all the warnings about terrorism, law enforcement authorities say the greater and more likely threat in this country comes from people like the suspect in the shooting of Rep. Gabrielle Giffords -- what are known as lone wolves.
And while some observers want to blame Saturday's bloodshed on lax gun laws or heated political rhetoric, experts say there is nothing more American than a loner who wants to be famous.
"If we feel that civility in public discourse is going to take away mass shootings we are mistaken," said Dr. Michael Welner, a forensic psychiatrist at New York University and an ABC News consultant. "Because the one common threat in mass shooting is, what does the shooter get out of it? And the shooter recognizes that if you assassinate a political figure you will be notorious."
"I think John Lennon had more to do with this than Sarah Palin," said Welner.
America has been plagued for decades by acts of violence attributed to lone wolves. The assassinations of the 1960's made household names of the men responsible.
The number of men, mostly young, who have sought similar notoriety since then only seems to have increased, up to this weekend's accused murderer, Jared Loughner.
"Most of them are very socially uncomfortable," said Brad Garrett, a former FBI profiler and special agent, "and so as a result they tend to withdraw and become more and more isolated and that is the reason I think we call them lone wolves because it's like they can't work in the pack, in other words they can't work in society."
Garrett, now an ABC News consultant, said he believes "there are thousands of people in this country like Mr. Loughner."
The loner who shot and killed Beatle John Lennon in 1980 is a textbook case.
The common theme with Mark David Chapman and history's other lone wolves is not ideology or politics or religion, according to Welner.
"The mass shooter will always justify [his actions] in some righteous or ideological way," said Welner, an ABC News consultant. "But at the end of the day, what drives that person is a sense that they high expectations for themselves. These are people with high self esteem."
Mass shooters act, says Welner, when they realize "those expectations are going nowhere. They recognize they will never amount to everything they dreamed of. And they choose something the public has given so much attention to and make a decision at some point, 'I can be larger than life.' And they make a decision to destroy the society around them."
In a prison interview, Chapman told ABC News' Barbara Walters why he killed Lennon.
"John Lennon fell into a very deep hole, a hole that was so deep inside of me that I thought by killing him, I would acquire his fame," Chapman told Walters.
The celebrity musician had somehow disappointed Chapman.
"And I see this real somebody who I perceived at that time to be a phony," said Chapman. "My nobody was wanting to strike down that somebody. I heard this voice saying over and over, 'Do it, do it, do it.' "
Said Dr. Welner, "He decided John Lennon was a fake. John Lennon's assassination gave him immortality."
The lone wolf shooters almost always leave behind signals, or now, on the internet, video statements.
Twenty-three-year old Seung-hui Cho mailed a tape to NBC News before he killed 30 students at Virginia Tech four years ago.