One woman was killed and five other people injured in a knife attack in central London on Wednesday night, Sky News reports.

Police were called to reports of an armed man attacking people in Russell Square, at around 10.33 p.m. (local time).

The woman who was killed was treated at the scene for her injuries but was declared dead a short time later. The condition of the other victims is not known.

A man was arrested at 10.39 p.m. after a Taser was fired by one of the police officers, reported Sky News.

Local police said the attacker had mental health issues, but did not rule out terrorism.

“Early indications suggest that mental health is a significant factor in this case and that is one major line of inquiry but of course at this stage we should keep an open mind regarding motive and consequently terrorism as a motivation remains but one line of inquiry for us to explore,” senior police officer Mark Rowley said in a statement quoted by AFP.

The incident comes after the Met increased the number of armed officers in London following the recent terror attacks in other countries in Europe. Attacks have hit Nice,Normandy and Wurzburg, Germany.

Wednesday night’s incident come just several days after the commissioner of the Metropolitan Police warned that a terror attack in the UK is a matter of “when, not if”.