Unknown attackers sneaked into a U.S. Air Force communications facility before dawn Friday and set off a bomb packed with nails, causing superficial damage but no injuries, a U.S. official said.

It was the second attack on a U.S. military target in Europe in less than 12 hours and came on the second anniversary of the U.S. bombing of Libya in retaliation for a discotheque bombing that killed an American in West Germany.The bomb, placed outside a maintenance building at the radio relay station near Madrid, went off at 6:15 a.m., U.S. embassy spokesman Robert Meade said.

"There were three U.S. personnel at the facility when the bomb exploded, but they were unhurt," Meade said.

The size of the bomb was not immediately known nor was the type of explosive used, Meade said, adding it "obviously" was meant to kill.

The station is in the town of Los Santos de la Humosa, 9 miles east of the U.S. Air Force base at Torrejon. Torrejon is 13 miles east of Madrid and houses three squadrons of U.S. F-16 fighters.

Meade said the attackers apparently avoided a guard patrol in the predawn hours, somehow got through the wire fence and placed the bomb.

"There is an hourly security patrol so they must have waited for it to pass before they entered," Meade said.

U.S. military personnel and Spanish police were checking the facility for other explosives.

It came on the second anniversary of the April 14-15, 1986, U.S. bombing of Libya, but Meade said there was no evidence to link it to the attack.

In Naples, Italy, police said Friday they believe a Japanese terrorist accused of attacking Western embassies around the world parked a bomb-laden car outside the Naples USO club, killing an American servicewom-an and four Italians.

Two Arab groups claimed responsibility for the explosion Thursday in a narrow street that runs past the the club, where a party was being held to welcome destroyer USS Paul to port. Seventeen people were wounded, including four U.S. sailors.

All five people killed were just outside the USO. They included Angela Santos, 21, of Ocala, Fla., who held the rank of radioman 3rd class and was based at the Naval Communications Area Master Station in Naples.

Libya's ambassador to Italy denied involvement in the bombing by Col. Moammar Gadhafi's government.

Italian investigators said Friday that Junzo Okudaira of the Japanese Red Army, which has links to terrorists in Lebanon, was the prime suspect in the Naples attack. An international arrest warrant was issued for him Friday.

Romano Argenio and Ansoino Andreassi of Italy's anti-terrorist police said they believed Okudaira, 39, had accomplices. They said both claims of responsibility were being taken seriously.