LONDON — A 19-year-old Norwegian man was arrested on suspicion of murder after a knife attack in Central London that killed an American woman and wounded five people, the Metropolitan Police said on Thursday.

The attack on Wednesday night, on the eastern corner of Russell Square in Bloomsbury — a neighborhood known for its handsome garden squares that is home to the British Museum and several universities — immediately raised fears about terrorism. But after investigating through the night, the police said there was no evidence that the attack was politically motivated.

“Whilst the investigation is not yet complete, all of the work that we have done so far increasingly points to this tragic incident as having been triggered by mental health issues,” Assistant Commissioner Mark Rowley, the top counterterrorism official at the Metropolitan Police, said in a statement. “At this time we believe this was a spontaneous attack and the victims were selected at random.”

The police had said in a statement early Thursday morning that terrorism was “one line of inquiry being explored,” but they later backed away from that theory.