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. Until Friday, he 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.
Previously enacted legal restrictions on guns might have made it more difficult for Holmes to buy certain weapons and kill so many people, but he still would have been able to purchase a gun.
In 1994, Congress approved a 10-year ban on 19 types of military-style assault weapons. This ban would have prevented Holmes from purchasing one of the four firearms police found on him and in his car — the assault rifle. The ban also would have prevented Holmes from buying new high capacity ammunition magazines so that he could fire off more shots before without having to reload.
But that law expired in 2004, and it's been more than a decade since gun control advocates had a realistic hope of getting the type of legislation they seek, despite predictions that each shocking outburst of violence would lead to action.
There is a law that restricts the purchase of body armor, but that's only if the purchaser is a violent felon. It's enforced through an honor system, as no background checks are required, said Dan Vice, senior attorney for the Brady Center to Prevent Gun Violence.
Holmes also "booby-trapped" his apartment with trip wires and explosive devices set up to kill, police said. He appeared to use three types of explosives — jars filled with accelerants, chemicals that would explode when mixed together and more than 30 "improvised grenades," according to a law enforcement official, who spoke on condition of anonymity, citing the ongoing investigation. It's unclear whether those materials were legally purchased or whether they were of such high quantities that purchasing them could have set off a red flag for an observant store clerk.
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.
In an intelligence bulletin, the FBI and Homeland Security Department reminded law enforcement about indicators of suspicious activity at entertainment venues such as movie theaters. Among those are significant changes to appearances from one visit to the next, missing hands or fingers, strange odors or bright stains on clothes, clothing that's not appropriate for the season, an unusual interest in security procedures and loitering without an explanation.
When people first saw Holmes in the theater, he was a silhouette, taken by some in the audience for a stunt that was part of one of the summer's most highly anticipated films.
But then, authorities said, he threw gas canisters that filled the packed the theater with smoke and opened fire as people screamed and dove for cover.
"This does keep you up at night," Ahearn said. "There's nothing you can do to predict that type of crime."