WASHINGTON (AP) - The lone gunman keeps law enforcement officers across the country awake at night.
He's hard to pick out of a crowd. He has no criminal record. Often, he hasn't told anyone about his plans. He's compiled a weapons cache legally. He doesn't show up on any law enforcement radar until after he's acted.
The government has been more successful stopping al-Qaida from pulling off another Sept. 11-type attack than it has in preventing deadly shooting sprees such as the one in the movie theater in Aurora, Colo.
Law enforcement officials say it's nearly impossible to stop someone like James Holmes, the intelligent 24-year-old who, officials believe, killed 12 people and injured dozens of others.
The threat of the lone offender has become such a concern that the FBI in 2009 created a more than 25-member task force to identify common behavioral traits and characteristics. In 2012 alone, there have been 22 mass shootings, according to the Brady Campaign to Prevent Gun Violence.
To stop al-Qaida, the government attacked terrorist hideouts, froze major sources of terrorists' funding and made it more difficult for them to acquire weapons and materials to build bombs.
Holmes does not appear to be part of any terrorist or criminal network; officials say his purchases were legitimate and raised no red flags. "There will be no easy or quick answers and maybe there will never be any answers," the Aurora police chief, Dan Oates, told CBS' "Face the Nation," adding that all the evidence gathered so far indicates "he wasn't particularly aided by anyone else."
Until Friday, Holmes did nothing to bring him to the attention of law enforcement.
"There's no way you can prevent it. There's absolutely no way," said Peter Ahearn, a former FBI agent. "It was random. It happened. There was nothing that could have prevented that unless someone saw him loading his car with guns."
The Department of Homeland Security runs a nation-wide "If You See Something, Say Something" campaign. Ads encouraging people to report suspicious activity are displayed around the country, including in some movie theaters, the department said.
Even so, Holmes arrived at the theater dressed in black, outfitted in a gas mask, ballistic helmet, vest and leggings, black tactical gloves and protectors on his throat and groin. He was armed with an assault-style rifle, a shotgun and Glock handgun.
So far, law enforcement has not determined a motive for the attack, and no one has come forward to say they saw Holmes doing something suspicious.
People have described him as clean-cut, studious and quiet. A man who had a drink with him just days before his deadly rampage said Holmes had a backpack and geeky glasses.
Holmes broke no laws when he purchased his weapons, and he passed the required background checks.
Many lone gunmen and lone wolf terrorists are intelligent and have normal cognitive functions, such as the ability to remember things, pay attention and concentrate, said Kathleen Puckett, a former FBI behavioral expert. Puckett did a study in 2001 for the FBI that examined lone offenders, terrorists and shooters. "Basically there was no common denominator outwardly that you could see," she said.
This leaves any hope of stopping someone like Holmes to the average person to say something if he sees something suspicious, said Christopher Voss, a former FBI agent and hostage negotiator. It's on the government to reassure people that they're not being paranoid, or overreacting, when they see something that doesn't seem right, he said.
"That's really the only defense against lone gunmen - for someone to have said something when they wrote off their observations and instincts," Voss said.