An attacker stabbed to death a woman in her 60s and injured five others near London’s touristic Russell Square on Wednesday. A 19-year old suspect has been arrested in relation to the attack while two of the wounded remain in hospital.

In a statement on Thursday, an official from London’s Metropolitan Police said that “early indications suggest that mental health was a factor in this horrific attack.”

An earlier statement had indicated that “terrorism remains one line of inquiry being explored.” British intelligence agency MI5 has categorized the terror threat level in the country as “Severe” since August 2014.

London police reported on Wednesday that it will dispatch an additional 600 police officers across the city.