Experts say increased vigilance by average Americans can help thwart future terror attacks.

Ken Gray spent 24 years working with the FBI's counterterrorism operations and now works at the University of New Haven.

He says people shouldn't rule out an attack in Connecticut similar to the one in New York City Tuesday.

"Any large gathering of people that is close to an accessible road becomes a possible target," Gray says. "So malls, sporting events, popular walking areas, those are all possible soft targets."

In Tuesday's attack, the suspect lived in New Jersey and rented the truck he used as a weapon there. Then he drove to Manhattan and slammed into numerous people in a bicycle lane.

"The problem with soft targets is that as soon as you put in a defense in one soft target, then something else becomes a soft target," he says.

Gray points to the case of Bridgeport man Faisal Shazad, who tried to blow up a car in Times Square seven years ago. 

"In that particular case, he built a bomb," Gray says. "He bought a car over Craigslist in Stratford, drove it down to New York, lit the fuse, and then left."

Gray says adding barriers to sidewalks can help prevent future car attacks -- but malls, sports venues and train stations still make easy targets.

To protect themselves, people should be aware of their surroundings and suspicious of strange behavior, Gray says, pointing to the recent release of FBI files that showed Sandy Hook shooter Adam Lanza had talked about his obsession with mass killers online.

"If you become aware of somebody's intent because they're acting suspicious, or they're talking about something like that -- if you see something, say something," Gray says.