German police have rushed to a vocational school in the western town of Menden after an alarm was triggered indicating a possible armed attack.
A spokesman for Menden police said a large number of officers had been dispatched to the Hoenne vocational school after police were alerted to a possible incident.
So far officers had found no evidence of an attacker or any injured people, spokesman Marcel Dilling said.
However, Mr Dilling added that "nothing can be ruled out at this stage" and the police operation was ongoing.
He said a portable alarm had been triggered from inside the school.
Eiko Pate, a journalist at local broadcaster Radio Hellweg, said students at the school had been told to lock themselves inside their classrooms. A police helicopter was seen circling the site, he said.
Germany has seen a number of shootings at schools over the years. In March 2009, a 17-year-old student killed 15 people and then himself in the town of Winnenden.