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.