KARLSRUHE, Germany — The pistol used to kill two U.S. airmen at Frankfurt airport jammed with six rounds left, possibly saving the lives of other American troops on their way to Afghanistan, a German investigator said Friday.

Two other U.S. Air Force personnel were wounded in the attack Wednesday by 21-year-old Arid Uka, an ethnic Albanian from Kosovo who had lived in Germany for years.

Uka was formally charged Thursday with two counts of murder and three counts of attempted murder. He refused to speak in court but made lengthy statements to police shortly after the attack, prosecutor Rainer Griesbaum told a news conference.

Uka told authorities he had taken the FN 9mm pistol and two knives to the airport specifically to kill Americans "as revenge for the American mission in Afghanistan," Griesbaum said.

He said a YouTube video Uka saw the day before the attack, allegedly showing a U.S. raid on a home in Afghanistan, had inspired him to prevent "further cruelties."

When he saw the busload of 16 airmen parked outside Terminal 2 heading to the nearby Ramstein Air Base, he approached one standing outside and, under the pretext of asking for a cigarette, asked if they were on their way to Afghanistan.

"When he said yes, he shot the 25-year-old serviceman from behind in the head," Griesbaum said.

Uka then stormed onto the bus, yelling "Allah Akbar!" — Arabic for "God is great!" — and shot the driver in the head, killing him as well, Griesbaum said.

He shot twice at a 25-year-old airman seated on the bus, wounding him, then fired once at a 21-year-old airman, wounding him as well, Griesbaum said.

"Then he tried to shoot a 22-year-old," Griesbaum said. "He pointed his pistol at his head and pulled the trigger twice, but the pistol jammed and no shots came out."

Uka fled, and the airman he had tried to shoot chased him down in the airport terminal and overpowered him, along with police.

The U.S. Air Force identified the two slain airmen as 25-year-old Senior Airman Nicholas J. Alden of South Carolina, and Airman 1st Class Zachary R. Cuddeback of Virginia.

Alden was assigned to the 48th Security Forces Squadron at RAF Lakenheath in England. Cuddeback was assigned to the 86th Vehicle Readiness Squadron at Ramstein Air Base in Germany.

None of the airmen were in uniform, except for the driver.

It was not known when or where Uka obtained the gun.

German authorities believe that Uka recently became radicalized and acted alone — but Griesbaum said the incident shows there is a serious danger from what he called "virtual Jihad" even if extremists are not part of a radical network.

"It underscores the danger of Islamists acting alone," he said. "There are no immediate measures against this."

Police said Uka was shy and had few friends, and appeared to get his ideas not from mosque attendance or personal contact with extremists but from the Internet. Uka had not been observed at any of the mosques that German police keep under surveillance, they said.

"This case shows anew that it is of paramount importance to even more intensely fight the jihadi propaganda on the Internet that was probably the trigger here," he said.

Uka was Facebook friends with a number of known extremists, including a Frankfurt imam who is currently under investigation on allegations he has been trying to recruit young men to fight in Afghanistan. Authorities say, however, there was no known contact between the two beyond being the social networking site.

In addition to contacts with radicals on Facebook, when Uka was aged 9 to 15 years old he lived in the same building as Rami Makanesi, a known extremist who was picked up in Pakistan last year and extradited to Germany where he faces charges of membership of a terrorist organization

It was not clear, however, how much contact Uka had with Makanesi, who left Germany in 2009 for Pakistan. Authorities noted that at the time they lived in the same building, Makanesi had not yet become radicalized himself.

Police said they have not been able to identify the video Uka said had inspired him to violence, or whether such a video even exists.

The U.S. has some 50,000 troops stationed in Germany. It operates several major facilities in the Frankfurt region, including the Ramstein Air Base, which is often used as a logistical hub for operations in Afghanistan and Iraq.


David Rising and Juergen Baetz contributed to this report from Berlin.