A Soviet-built fighter jet flown into Key West by a defecting Cuban pilot apparently wasn't detected until it began circling around a Navy base. U.S. air defense officials want to know why.

The pilot, Maj. Orestes Lorenzo Perez, flew a MiG-23 to the Key West Naval Air Station on Boca Chica Wednesday."The pilot tried to radio the tower using an international frequency, but he could not come up with it," according to a Navy source who spoke on condition of anonymity. "When the tower spotted the plane, they tried to contact him."

No U.S. planes took off to intercept and escort the Cuban jet, which is standard procedure when an unknown plane is detected, said Maj. Thomas Niemann, a spokesman for NORAD, the nation's air defense agency in Colorado.

"We didn't launch any interceptors. Why we didn't is a question people are reviewing throughout the NORAD system right now," Niemann said.

The Defense Department referred questions to NORAD.

NORAD has not determined if radar picked up the MiG at any point during its flight, Niemann said, but he said it might have taken as little as eight to 10 minutes to fly the 90 miles from Cuba to Key West.

If the plane came in at only a few feet above the sea level, Navy aerostat balloons and other radar equipment could have been confused by choppy waves, according to military experts.

Lt. Karl Johnson of the Naval air command headquarters in Norfolk, Va., said the plane, which the Navy initially identified as a MiG-27, was now believed be an older modified MiG-23. The difference between the two models is very slight, he added.

Perez was taken into custody by immigration officials after U.S. pilots gave him a welcome bag including candy bars, potato chips and a six-pack of beer, base personnel said.

State Department spokesman Douglas Gray said Thursday the MiG will remain in a hangar until U.S. and Cuban officials negotiate its return.