German police 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 persons, the spokesman Marcel Dilling, told German station n-tv.
Dilling added, though, 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.
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