A large contingent of the protesters Chicago will see this week have gotten permits for their demonstrations and have stressed their gatherings will be peaceful. But there's another group of people who are likely planning spontaneous actions, and it's those fringe, potentially violent protesters who are causing concern among summit organizers.
Efforts are ongoing, they say, to find out how and when they might strike. Those efforts include monitoring social media, which is alive with talk of protests.
"If there's any information there that's suggesting doing anything that's potentially illegal, then [officials are] taking a little more aggressive look," said Arnette Heintze, the former head of the Secret Service in Chicago who's acting as a security consultant for the Official NATO Host Committee.
Much of the Internet chatter is among lesser-known, often anonymous protesters, like the ones who last fall purposefully disrupted larger, otherwise peaceful demonstrations in Rome with violence.
"The first half hour they bombed, fire-bombed a car, and then within another 20 minutes they did two more cars," said DePaul University School of Music professor Cathy Elias, who found herself caught up in the middle of the melee.
Chicago police insist officers have undergone months of special training to be able to handle whatever happens.