Lambert Field officials cleared the A concourse Friday night and unloaded passengers from five planes to be rescreened after a security worker spotted what looked like a gun passing through an X-ray machine.
The new screenings and a search of the planes did not turn up the object.
A sixth plane that took off at the time was diverted to Detroit so passengers aboard that plane could be rescreened later in the evening.
At about 6:30 p.m., a Transportation Security Administration screener saw an image on a X-ray monitor that resembled a double-barrelled handgun, officials said.
Screeners called airport police, but the suspicious item and the man who brought it through security left the screening area.
Authorities cleared the A concourse to search for the possible weapon. About 300 people in the concourse or on the five planes still parked at their gates were brought out to be screened again. The planes were also searched.
An Air Canada flight headed to Toronto that had already taken off was sent to Detroit because that was the nearest airport, officials said.
A TSA official said the agency was investigating how sure the screener was that the item was a gun and why the man was not stopped at the checkpoint.
Airport Police Chief Paul Mason said that when a suspicious item is spotted, the bag containing the item is supposed to remain in the X-ray machine until police arrive.
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