A man stabbed passengers with a knife on a regional train near Aachen on Friday morning, injuring at least three people.
A federal police officer who happened to be sitting on the train and who was on his way to his office overpowered and arrested the man, the Cologne police said.
The background to the crime and the identity of the attacker were initially unclear.
According to initial investigations, the man had pulled a knife on the Euregiobahn RE4 around 7.40 a.m. shortly after leaving the Herzogenrath station and cut fellow passengers. Three people are now being treated in hospitals, there is no danger to life.