The head of the Police Department's counterterrorism bureau was detained by federal authorities Thursday after trying to board a flight at Los Angeles International Airport (search) with a loaded handgun in his carryon luggage.

John Miller (search), an ABC-TV reporter before he was hired for the security job, was off duty and heading to New York with his family when Transportation Security Administration officials discovered the .38-caliber handgun in his bag, TSA spokesman Nico Melendez said.

Miller is authorized to carry the weapon, but failed to disclose he was traveling with the gun.

Police Chief William Bratton said he spoke with Miller, who was "extraordinarily embarrassed" by the incident.

"It's a human mistake that was made," Bratton said. "John is certainly wishing it didn't happen.

Miller was briefly detained before he and his family were permitted to board their scheduled flight. The gun was confiscated.

"He knows the airport procedures. ... He didn't realize he had it," said Lt. Horace Frank, an aide to Miller. "It was an oversight."

Miller was on his way to New York to tape a farewell message to ABC's Barbara Walters (search), who is retiring. Before joining the LAPD, Miller co-anchored the network's "20/20" news magazine with Walters.

Bratton, who hired Miller as his spokesman when he was New York City police commissioner, then recruited him to come to Los Angeles, ordered an administrative investigation. He said he had no doubt that Miller made a simple mistake.

Under federal rules, Miller could be fined up to $3,000.