UPDATE: The woman killed has been identified as an American.

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One woman is dead and five people were injured when a 19-year-old man attacked bystanders in London's Russell Square with a knife.

"A man was arrested at 22:39 hours and a Taser was discharged by one of the arresting officers," police said.

The man is 19 years old, but his name was not released. At a press conference early Thursday morning, Metropolitan Police Asst. Commissioner Mark Rowley said, "This was a tragic incident resulting in the death of one woman and five others being injured. Early indications suggest that mental health was a factor in this horrific attack.

Authorities are not ruling out terrorism.

"However we are keeping an open mind regarding the motive and terrorism remains one line of inquiry being explored," he said.

The female victim was in her 60s and treated at the scene but was pronounced dead shortly thereafter.

"As a precautionary measure, Londoners will wake up this morning to notice an increased presence on the streets of officers, including armed officers today. We would urge the public to remain calm, alert and vigilant," said Rowley. 

An anonymous source described the attack: the "guy kept screaming his guts out" and "police were screaming 'stop, stop, stop.'" The attacker was on the ground "for at least 30/40 minutes."

Zuhair Awartani was trying to return to a hotel where he was staying when he saw a "dark-skinned man" in his "late 20s" being arrested. 

London Mayor Sadiq Khan police are "working extremely hard to investigate exactly what has happened and to keep the rest of us safe".