A top security official says the suspect in the murder of two U.S. airmen at the Frankfurt airport has confessed to targeting the U.S. military and says he acted alone.

Hesse Interior Minister Boris Rhein told reporters Thursday in Wiesbaden the suspect, identified as 21-year-old Arif Uka from Kosovo, was apparently radicalized over the last few weeks by looking at Islamist websites in Germany. U.S. officials confirm that the suspect was extremely active on the Internet, specifically Facebook. They say they are looking for any potential contacts with the American cleric Anwar al-Awlaki, who is seen as the new generation of digital jihadist.

The U.S. Air Force released the names of the victims on Thursday: Senior Airman Nicholas J. Alden , 25, of Williamston, S.C., and Airman First Class Zachary R. Cuddeback, 21, of Stanardsville, Va.

U.S. officials tell Fox News that German authorities are investigating "substantial evidence that Uka has links to Islamic fundamentalist groups in Germany." Officials also say evidence supports the view so far that Uka did act alone -- which he stated -- but no final conclusions have been reached.

A U.S. law enforcement official says the suspect was not on any American terrorism watch list.

Rhein says that the investigation indicates that he acted alone and did not belong to a terrorist network or terrorist cell, but German federal prosecutors say the attack appears to have been motivated by Islamic extremism.

Officials say the shooter was able to physically board and the bus was clearly marked as U.S. Air Force.

Prosecutors said in a statement Thursday that "because of the circumstances there is a suspicion that it was an Islamist motivated act."

The federal prosecutors office took over the case from local officials early in the day and are working with Frankfurt and federal police, as well as American investigators.

The suspect, Arif Uka, is said to be a Muslim man with family in Kosovo who was born in Germany and was working at the airport at the time of the attack. He reportedly said "Allah Akbar" before opening fire on a bus outside the airport, a source told Fox News. The deadly attack also wounded two others before the man was taken into custody.

American officials in Washington told the New York Times that there was no immediate evidence tying the gunman to a terrorist organization. German police originally said that they had no such evidence either, although they didn’t rule out the possibility of a link.

But a member of the U.S. House Homeland Security Committee, Rep. Patrick Meehan, said in Washington that it looked like a terrorist attack, though he noted that he still was being briefed on the details of the shooting.

A senior U.S. military official also told the Wall Street Journal that the suspect has “some kind of Islamic ties,” but was unsure how deep they were or if they were aligned with Al Qaeda or local terrorist groups in Kosovo.

"We don't think they are particularly well-organized, but they are present," the official told the Wall Street Journal.

Police said the attacker had an altercation with U.S. military personnel in front of a bus outside the airport's Terminal 2. They said the man started shooting, then boarded the bus briefly and was apprehended by police when he tried to escape.

Frankfurt police spokesman Juergen Linker told the DAPD news agency that one airman remained in critical condition after being shot in the head. The other airman was not in life-threatening condition, Linker said. Both men were being treated at the Frankfurt University clinic. 

U.S. Air Force Europe spokeswoman Maj. Beverly Mock said all four victims were airmen.

They were all based at the Lakenheath military base in Britain.

A military official in Europe says the airmen were going to be deployed to either Iraq or Afghanistan.

President Obama said he was "saddened and outraged" by the shooting.

The president says the U.S. will spare no effort in finding out how the "outrageous" act took place and will ensure the perpetrators are brought to justice. He called it a tragedy.

In Berlin, German Chancellor Angela Merkel said her sympathies were with the victims and their families, and pledged that Germany would do everything in its power to investigate the crime.

"It is a terrible event," she said.

The U.S. has drastically reduced its forces in Germany over the last decade, but still has some 50,000 troops stationed here. It operates several major facilities in the Frankfurt region, including the Ramstein Air Base often used as a logistical hub for operations in Afghanistan or Iraq.

At the airport, taxi cab driver Salimi Seraidon said he was sitting at a stand about 200 yards away when the attack took place and said it was over quickly as police rushed onto the scene.

"We just heard the shots," he said.

The American forces in Germany have been targeted in attacks in the past, including a 1986 bombing at a disco in then-West Berlin that was known to be frequented by U.S. servicemen.

Two soldiers and one civilian were killed, and 230 others injured in that attack, which a Berlin court in 2001 ruled was organized by the Libyan secret service and aided by the Libyan Embassy in then-communist East Berlin.

The leftist terrorist Red Army Faction was also responsible for a string of attacks on Americans in the 1970s and 1980s before the group was disbanded in 1998.

More recently, German police thwarted a plot in 2007 to attack U.S. facilities by members of the extremist Islamic Jihad Union. Four men had planning to attack American soldiers and citizens at facilities including the U.S. Air Force's Ramstein Air Base in Germany but were caught before they could carry out the plot.

The Associated Press contributed to this report.