It played out like a movie.

The FBI says a passenger about to board a plane for Phoenix at Philadelphia International Airport yesterday saw an airline employee switch carry-on bags with another passenger - after they had passed through security - and got suspicious.

The observant passenger watched as passenger Damien Young, 29, approached the US Airways customer-service counter at gate C-28 just before 7 a.m. and saw Young pass a black, laptop-style bag he had been carrying to Roshid Milledge, a US Airways customer-service representative, according to an FBI affidavit.

Milledge, 38, who was in uniform, handed a similar-looking laptop-style bag to Young, the other passenger said.

The passenger, unaware one of the bags contained an unloaded handgun, according to authorities, continued to observe Young and Milledge and told authorities he saw Milledge acting "fidgety" and nervous.

The passenger then alerted US Airways security.

Passengers were on board when a US Airways manager entered the plane and questioned Young.

Young denied switching bags and the plane started to taxi down the runway. Security then alerted the pilot to stop, and a US Airways security official and a Philadelphia police airport officer climbed aboard on the tarmac.

An unloaded silver and black Smith & Wesson 9 mm semiautomatic handgun was confiscated from Young's bag, the affidavit said.

The gun was registered to Young and he has a valid Pennsylvania permit to carry it, according to the affidavit.

Young and Milledge, who had been roommates on Ringgold Street in South Philadelphia, were arrested for violating federal laws about carrying weapons aboard aircraft, authorities said.

Janice Fedarcyk, special agent in charge of the FBI's Philadelphia office, called the incident "an isolated incident, involving only these two individuals."

Milledge told authorities he had grabbed the wrong bag, the one belonging to Young, when he left for work yesterday morning, and was simply trying to return it to Young when the bags were switched. Young was moving to Phoenix, according to the affidavit.

Young told FBI agents that he asked Milledge how to take the gun with him and Milledge said he would assist him in circumventing security-screening procedures by carrying the gun in through an employee entrance. *