Police in Germany have helped the US military secure an air base after a bomb threat was made by telephone.
German police said they "moved swiftly" to protect the base after US forces informed them of the threat.
The call to the US base at Spangdahlem, near Trier, was made on Monday, one day before the sixth anniversary of the 9/11 attacks on America.
Last week, German police said they prevented attacks being planned against targets including US military forces.
Two senior US intelligence officials say a controversial wiretapping programme helped disrupt the plot in Germany as well as another attack allegedly being planned in Denmark.
German police arrested three suspects and said they had been planning a series of simultaneous car bomb attacks on a variety of targets including US military bases and nightclubs frequented by American citizens.
Another target was reported to have been Frankfurt airport, one of Europe's busiest air hubs.
The US director of intelligence, Michael McConnell, told a Senate committee that eavesdropping had revealed that the suspects had obtained explosive liquids.
He said Congress should not restrict the programme. A temporary bill was passed in August following heated debate, allowing eavesdropping on foreign terror suspects without a warrant.
Police in Trier said Monday's threat was made by a man speaking German with a Russian or Turkish accent.
He called a publicly listed number at the Spangdahlem air base on Monday evening.
US F-16 fighter jets are based at Spangdahlem
The caller said he had at least four accomplices, according to a police statement.
"After the telephone threat, the American forces informed the police, who moved swiftly with US security forces to protect the air base," said the statement.
The police are looking for the anonymous caller.
The base at Spangdahlem is home to the 52nd Fighter Wing of the US Air Force, equipped with F-16 jet fighters which are used in US military missions around the world.