James Holmes. Photo from University of Colorado Denver
WASHINGTON (AP) -- Law enforcement officials say lone gunmen like the suspect in the Colorado theater massacre are the stuff of bad dreams, because they're generally invisible until it's too late.
Officials say such attackers often have no criminal record, keep quiet about their plans and acquire weapons legally.
Law enforcement officials say it's nearly impossible to stop someone like James Holmes, the 24-year-old former graduate student who's accused of killing 12 people and injuring dozens of others.
The threat of the lone offender has become such a concern that the FBI in 2009 created a task force to identify common behavioral traits and characteristics.
The Brady Campaign to Prevent Gun Violence says this year there've been 22 mass shootings.