Dortmund, Germany - German police sealed off a wide area around Cologne Central Mosque on Tuesday after a threatening email was received.
"The content had such a high threat potential that we had to act immediately," a police spokesman said without giving details of the email's content.
The mosque, the largest in Germany and one of the biggest in Europe, was evacuated.
A spokesman for the Turkish-Islamic Union for Religious Affairs (DITIB), Germany's largest Muslim umbrella organization which runs the mosque, told dpa that it was a bomb threat.
A high number of specialist officers had been searching the mosque complex in the city's Ehrenfeld district since the morning, but had so far "found nothing that poses a danger," the spokesman said.
The operation was gradually wound down after one and a half hours.
The DITIB spokesman said that the evacuation had gone smoothly. How many people were in the complex at the time was unclear.
In addition to employees, many visitors come and go every day. Two school classes had visited in the morning, he said.
"We are worried, we are afraid we are experiencing a concentration of attacks on mosques," DITIB spokesman Zekeriya Altug said. "A bomb threat has a new quality."
Investigations have been launched into the crimes of disturbing the public peace by making threats.
The controversial Cologne Central Mosque was officially opened last September by Turkish President Recep Tayyip Erdogan.
Politicians at the regional and federal level have criticized DITIB, a branch of the Directorate of Religious Affairs in Ankara, for being an arm of the Turkish state and too close to the increasingly authoritarian Erdogan.dpa