Phillip Lyons, executive director for the Center for Policing Innovation and a professor at Sam Houston State University, agrees.
"A lot of times, with 20/20 hindsight, people knew there was something suspicious," he says. "So the recommendation we would give ordinary folks is, if something looks out of the ordinary, if something is raising red flags for you, report it. If it turns out to be nothing, no harm, no foul."
For example, a few people knew that one of the 9/11 terrorists was taking flying lessons to learn how to take off but not land, Lyons says. In hindsight, that detail could have been invaluable to authorities.
Many think that community policing is more important now than ever, he says, and he's seen it work: Some mosques, he says, have notified the police when members seemed to be in the process of being radicalized. But they only do that if they trust the police, which, in turn, happens when the police understand all the people in their communities.
Although the signs that somebody is being radicalized are subtle, they may include anger, often against the government or American society, spending a lot of time online, spending time in groups of people who are similarly angry, and inexplicably losing interest in things they had been interested in before, Lyons said.