Now as large numbers of tourists flood into New York City and the nation's capital, the FBI is warning of a new terror tactic.
Citing an FBI informational document, ABC News reports a so called "battle of suspicious bags" is being encouraged on a jihadist website.
Bomb expert Kevin Barry with 20 years experience in the NYPD says the bags could be filled not with bombs, but with innocuous items like water bottles or socks.
The potential terrorists would be watching the response.
"How they evacuate. The standoff distances. How long it takes additional emergency services to come to the scene," Barry said.
While not giving away details, police do have numerous ways of responding to these types of incidents.
The report says no evidence of a suspicious bag campaign has been found so far in New York City or Washington.
"After every major incident there will be a spike in calls because people become more nervous and there is more reporting of suspicious items," Barry said.
Putting more emphasis, security experts say, on the slogan "see something, say something."
"People are becoming more suspicious, more vigilant and that results in more calls. We understand that happens and we are prepared to respond," police commissioner Ray Kelly said.
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