KEY WEST, Fla. (AP) _ A Cuban air force captain asked for political asylum after flying his unarmed MiG jet to the United States, the second such defection in less than three years.

The MiG-21 landed at the Key West Naval Air Station on Boca Chica Key Friday afternoon, sparking a flurry of worried and often conflicting statements from U.S. officials about how and if the jet eluded radar detection.

The pilot, Capt. Enio Ravelo Rodriguez, 32, was turned over to the Immigration and Naturalization Service. He remained in custody late Friday.

Cmdr. Drew Malcomb, deputy director of public affairs for the North American Air Defense Command (NORAD) and U.S. Space Command in Colorado Springs, Colo., said the plane flew too low to be seen by radar.

''It's something that concerns us very much, obviously,'' Malcomb said. ''After all, someone has made an incursion into the sovereign territory airspace of the United States.''

It was spotted at an altitude of 50 feet traveling more than 500 mph, Malcomb said.

But Morgan Smith, commander with the U.S. Atlantic Fleet in Norfolk, Va., said the MiG was picked up by radar for about 4 minutes as it was coming in.

The single-seat jet was equipped with a .22 millimeter machine gun, but had no missiles, Smith said.

Sen. Bob Graham, D-Fla., called for a Pentagon investigation.

''Now is not the time to let our defenses down against (Cuban President Fidel) Castro,'' Graham said. ''It is especially important at this time, when Castro is increasingly desperate, to remain vigilant.''

Former Cuban air force pilot Orestes Lorenzo landed a MiG at Key West in March 1991. Last December, he made a celebrated rescue flight in a small plane, picking up his family from Cuba last December. Ten days later, a Cuban pilot diverted a domestic flight to Miami with 53 people aboard.

Nearly 2,000 Cubans have floated to Florida so far this year in boats, makeshift rafts and inner tubes as the deprived Cuban economy has relied increasingly on rationing food, power and fuel.