A man that set off an alarm on Monday by entering a school in a town near Stuttgart with a firearm is still being sought by police after another man was mistakenly detained, German police said.

Police tweeted that the man they arrested was let go again and that the suspect had probably fled on a motor bike.

Spokesman Bjoern Reusch said the suspect didn't use his weapon and nobody was hurt when he briefly showed up inside the school building in Esslingen, near Stuttgart.

The Friedrich Ebert School was under a security lockdown Monday and all students had to stay inside their classrooms while officers searched the building.

Police described the suspect as "male, about 180 cm tall, slim, (wearing a) red t-shirt, blue jeans, camouflage cap worn backwards, a three-day beard, reading glasses, dark complexion."