The Westminster attack suspect was British and known to MI5, the Prime Minister announced today.
"His identity is known to the police and MI5, and when operational considerations allow, he will be publicly identified," said Theresa May.
"What I can confirm is that the man was British-born and that – some years ago – he was once investigated in relation to concerns about violent extremism. He was a peripheral figure."
May's remarks were made as part of her speech to the House of Commons, where several MPs were locked in yesterday afternoon during the violent attack which left four people dead and dozens injured.
May outlined the details of yesterday's attack, and paid tribute to PC Keith Palmer who was stabbed to death.
"PC Palmer had devoted his life to the service of his country," she said.
"He had been a member of the Parliamentary and Diplomatic Protection Command for 15 years, and a soldier in the Royal Artillery before that.
"He was a husband and a father, killed doing a job he loved. He was every inch a hero. And his actions will never be forgotten."