Police said the attack was not linked to terrorism, and that mental health appears to have been a factor.
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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.
But Rowley said terrorism isn't being ruled out. "However we are keeping an open mind regarding the motive and terrorism remains one line of inquiry being explored," he said.
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."
Police officers and London Ambulance Service attended to the scene. Additional police units have been deployed to the area to "provide reassurance," police said.
The female victim -- who was in her 60s, police said -- was treated at the scene but was pronounced dead a short time later.
Large police presence around Russell Square, allegedly due to gang violence... pic.twitter.com/7RnDwFnhq6— Jeremy Cheng (@jezze_0410) August 3, 2016