And as more information is released - of course he was

From the UK Telegraph:

London terrorist had twice been referred to police over his extremist views
One of the three jihadists who murdered revelers in central London on Saturday had been reported to the anti-terror police on at least two occasions, it has been claimed.

A former friend of the terrorist, who was shot dead by police along with two accomplices, claimed he had been radicalized while watching YouTube videos and said he contacted the authorities ­after becoming concerned over his friend’s extremist views.

A neighbour also claimed she had contacted police in Barking, east London, after the suspect tried to convert her children to Islam and radicalize them. The man is not being named at the request of the police.

But we are supposed to be tolerant of others.