American Woman Killed in Knife Attack in London, 5 Others Injured

A 19-year-old man was arrested, police said.

— -- Police presence on the streets of London increased today after an American woman was killed and five other people were injured in a knife attack in London's Russell Square Wednesday night, authorities said.

Metropolitan Police Asst. Commissioner Mark Rowley said this morning, “At this time we believe this was spontaneous" and the victims were "selected at random." There was no evidence of radicalization or terrorism as a motivation, Rowley said.

The American woman killed was in her 60s. Police did not say where in the U.S. she was from. The State Department confirmed the death of a U.S. citizen but did not release the individual's name.

Of the five injured victims, one remains in hospital today -- a British man who suffered a stab wound to his stomach, police said.

The other four victims -- an American man, an Australian woman, an Australian man and an Israeli woman, also suffered stab wounds but have since been discharged.

Armed police responded to the scene within six minutes Wednesday night and the suspect, a 19-year-old Norwegian national of Somali origin, was Tasered and taken into custody. No shots were fired.

Police said mental health issues appeared to have been a factor.

The mayor of London, Sadiq Khan, said, "Police have spoken to him and are seeking to establish the full facts including motives for this attack."

The investigation is ongoing. Officers have searched an address in north London today and will search another in south London.

Metropolitan Police's Rowley said today there is an "increased presence on the streets of officers, including armed officers."

"We would urge the public to remain calm, alert and vigilant," Rowley said.

ABC News' Darren Reynolds, William Gretsky and Rachel Katz contributed to this report.