There is a strong suspicion he was planning a criminal act, the public prosecutors' office in Dusseldorf said.

The teenager's lawyer had earlier demanded his release due to a lack of evidence.

The fear of what the state interior minister of North Rhine Westphalia, Herbert Reul, described as an "Islamist-motivated threat" led to a large police operation on Wednesday evening in the western city of Hagen.

The 16-year-old's father and two brothers were also detained by police.

The police operation came after what Reul said was a "very serious and concrete tip-off" regarding a possible attack on the Hagen synagogue.

The alleged threat came on Yom Kippur, the holiest day of the year for Jews, and led to the cordoning off of the areas around the synagogue and the cancellation of services there.

Searches at the 16-year-old's home did not reveal any bomb-making components.

Electronic media such as mobile phones and storage media were seized, which still have to be evaluated.

The youth is said to have had contact with a known Islamist abroad and discussed bomb-making, according to security sources.

According to the investigators, the youth admitted the contact but has denied an intention to attack the synagogue.

The original tip-off is said to have come from a foreign intelligence service but it is not clear which one.

Australian Associated Press