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."
Mark David Chapman
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.
"You have vandalized my heart, ripped my soul and torched my conscience," said Cho on the tape. "You thought it was one pathetic boy's life you were extinguishing. Thanks to you, I died like Jesus Christ to inspire generations of the weak and the defenseless people."
Brad Garrett says that Cho was able to go from room to room methodically shooting people, without changing expression, "because he's not shooting human beings, he's shooting things that have caused him pain and anger. He's going to seek revenge on them, and it's okay if you're dead because you don't count."
Law enforcement professionals and psychiatrists say another classic lone wolf trait involves sexual dysfunction.
George Sodini, 48, killed four women at a Pittsburgh health club after months of rejection by women he asked on dates.
"It is easy for me to hide from my emotions for one more day," said Sodini in a video posted on YouTube.
Welner: Portray Shooters as Losers
In Washington, D.C.,the great fear among law enforcement is of copycat lone wolves who may target other unguarded members of Congress.
But law enforcement officials say spotting a lone wolf before he can kill is highly unlikely.
Brad Garrett questions what can really be done to stop lone wolf attacks. "If you're talking about thousands and thousands of people out there," said Garrett, "it's a scary thought.
Garrett said lone wolves pose as great a danger as organized terrorists. "Maybe even more so," said Garrett, "because they're here, they're in our community. Maybe a few people see them acting inappropriately, but they're just doing their thing slowly and one day they erupt."
The suspect in Arizona was easily and legally able to buy his weapon at a gun store outside Tucson. Every country has unstable people, but not every country has such liberal gun laws.
Daniel Webster, codirector of the Johns Hopkins Center for Gun Policy and Research, told ABC News that controlling the supply of guns is key. "In order to lower the number of shootings and lethal shootings," said Webster, "whether they're mass shootings or not, you have to do something to address the availability of guns.
But Welner said that in his professional opinion, the easy availability of weapons did not play a role in the Tucson incident.
Welner says the real answer is to focus more on the heroes of the shooting, such as Congresswoman Gifford's intern Daniel Hernandez, who was recognized today in the Arizona legislature, and focus less on those responsible for the pain.
"It's an attention seeking crime," says Welner, "which is why the perpetrators of mass shootings should be remembered as rejects, losers, perverts, because then there won't be copies."
Said Welner, "That is a key message not just for the press. [It's] a message for teachers, it's a message for neighborhoods, and it's how we as a society can eliminate something that's distinctively American. "