The names of the dead were being withheld pending notification of the families. The injured were being treated at Frankfurt hospitals for wounds listed as severe in one case and serious in the other, the German police said.

Germany has so far been spared a catastrophic terrorist attack of the kind experienced in Britain and Spain. But three of the Sept. 11, 2001, attackers lived in Germany, and security officials acknowledge that the country remains a breeding ground for Islamic radicals.

Germany, Britain, and France have been on edge since last year over warnings of the possibility of pending attacks by radicalized European citizens who might have received training in the lawless border region between Afghanistan and Pakistan. In November, the German authorities beefed up security at train stations and airports after an informant said that a group linked to Al Qaeda was planning to shoot civilians in a public place.

Mr. Füllhardt said that the dead and wounded airmen were part of a group of 13 who had just arrived from London and had boarded the bus to go to the American military base at Ramstein, about 80 miles southwest of the Frankfurt Airport. He said they were in uniform and headed for deployment in Afghanistan. The dark blue bus had United States Air Force license plates, though American military officials regularly warn their personnel in Germany not to use military license plates to be less conspicuous.

A little before 3:30 p.m. local time, the suspect argued with some of the airmen and then shot one who was standing in the open door as well as the driver, Mr. Füllhardt said. The two airmen who were wounded were also near the door. The suspect fled and was captured by a federal police officer.

The attack took place at Terminal 2, near the check-in counter for the Russian airline Aeroflot. Military buses often use the location to pick up soldiers and airmen in transit via Frankfurt, which is one of Europe’s busiest airports.

In the aftermath of the shooting, the area was rapidly cordoned off, but the terminal continued operations. A police dog sniffed the area, searching for spent cartridges. Several bullet holes were visible in the windshield of the bus, which was eventually towed away. Two carloads of men and women who appeared to be American officials arrived, but they did not speak to reporters.