Police tactical units in the German city of Essen on Thursday carried out searches at two schools and an apartment after they received information that a 16-year-old male pupil might have been planning violent crimes at his current or former school.

Both schools were closed during the searches.

A spokesperson for the state prosecutor's office in Düsseldorf said the pupil, who was arrested in the early hours of Thursday morning, was now the subject of a terrorism investigation.

Joachim Stamp, the deputy premier of the state of North Rhine-Westphalia, where Essen is located, wrote on Twitter that a "suspected Nazi terror attack" had been foiled.

What do we know?

At a press briefing, the state's interior minister, Herbert Reul, said police had "possibly prevented a nightmare."

He said they had found explosive and bomb-building materials at the pupil's apartment, located near his current school, where he lived with his parents.

The materials were "functional but not yet ready for use," he added. 

Reul said there were indications that the pupil had psychological problems, saying notes had been found that could "be read as an urgent cry for help by a desperate young man."

The pupil appeared to be acting alone, he said.

Crossbow and explosives found

Reul said a crossbow with bolts, 16 lengths of piping, some of which contained nails or were fitted with timers, explosives and a homemade firearm were among the potential attack materials found at the apartment.

SS runes and numerous documents with far-right extremist, antisemitic and anti-Muslim content were also discovered, Reul added.

He said no explosives had so far been found at either school.

Reul and several media outlets reported that a fellow pupil had informed police of the attack plans.

tj/msh (AFP, dpa)