GRAPEVINE, Texas -- A security breach Thursday afternoon forced the evacuation of three terminals at Dallas-Fort Worth International Airport.

A security official says a man bolted through as he was being processed at a passenger checkpoint at about 3 p.m.

Transportation Security Administration spokesman Ed Martelle says the man slipped through somewhere into Terminal A after his bag failed a test by an explosive trace device.

"This was the explosive trace device, basically the screener takes a swab and runs it over the bag, and puts it in the machine which reads the cloth. That cloth tested positive for some kind of explosive material, we dont know what. But we have double-checked the machine, the test was a good test, the alarm was positive, we checked the calibrations and everything else...we believe that there was something on that bag that was very dangerous," Martell said.

Martelle says agents tried to detain the man, but "he just kept going." Martell added that because terminals A, B and C are connected, the man could have been on any concourse.

Martell said that the TSA is not concerned at this point about the bag being left in the concourse, saying a search has been conducted and no trace of the bag has been found.

Officials do not yet have a description of that man.

Officials also said the evacuation primarily affects American Airlines, the nation's largest carrier.

Officials shut down and evacuated terminals A, B and C, and began rescreening passengers and allowing some back into the airport at about 4 p.m.

There are four terminals in all at the airport.

One passenger told NBC 5 that the evacuation went smoothly, and that passengers calmly followed directions being announced over the intercom during the evacuation.

DFW is the third largest airport in the country.