Man arrested on suspicion of murder after London knife attack

A man has been arrested on suspicion of murder following a knife attack in central London in which one woman was killed and five people injured, police said on Thursday.

"Early indications suggest that mental health was a factor in this horrific attack. However we are keeping an open mind regarding the motive," London Metropolitan Police Assistant Commissioner Mark Rowley said in a statement. Police used a Taser electric shock gun while arresting the 19-year-old suspect.

The threat level in Britain remains at "severe", its second highest level, meaning a strike was "highly likely". London police had already promised to deploy more armed offices after a spate of deadly attacks in other European countries. Police chiefs and security bosses have repeatedly warned that Islamic State (ISIS) fighters want to carry out attacks against Britain, a close ally of the United States.

London police said on Wednesday they would deploy an additional 600 armed officers across the capital to protect against the threat of attacks. Rowley has previously warned that ISIS was seeking to radicalise vulnerable people with mental health issues to carry out attacks. He said that, in some operations, police commanders have taken advice from specialist psychologists.

London was hit by coordinated suicide bombings on July 7, 2005, when four Islamist extremists targeted three underground trains and one bus, killing 52 people. One of the bombs was detonated on an underground train travelling between Kings Cross and Russell Square.