Because of suspected explosives, the federal police searched an Intercity train in Wunstorf near Hanover with sniffer dogs.
The suspicion was not confirmed, but around 100 passengers had to wait a good hour at the station on Saturday evening before they could continue their journey towards North Rhine-Westphalia, as a spokesman for the federal police said on Sunday.
The "Hannoversche Allgemeine Zeitung" had previously reported on the operation.
The public prosecutor's office in Düsseldorf and the police in Cologne said on Sunday that a 17-year-old was being investigated on suspicion of preparing a serious act of violence that endangered the state.
The suspect has been arrested for the time being.
According to this, information from the personal environment of the 17-year-old from the greater Cologne area led to investigations into the possible risk situation in the Hanover region.
The federal police picked up the young people on Saturday afternoon at Hanover Central Station.
A defuser examined the suspect's bag, but found it contained no dangerous items.
According to the federal police, the 17-year-old had been in the IC before the control at the main station, which was later searched.
It was said that he behaved conspicuously and left a bag with objects at the travel center.
The area was then cordoned off, but there was only clothing in the bag.
The Hanover police said the operations were preceded by a note from the police in Cologne.
The local officials were informed about the state criminal investigation offices in North Rhine-Westphalia and Lower Saxony.
The Cologne state security has set up an investigation group.
The public prosecutor's office in Düsseldorf plans to provide more information on Monday.