Transcript for UK prime minister: London attacker was known to authorities, British-born
Mr. speaker the whole country will want to know who was responsible for this atrocity. And who measures that we are taking to strengthen our security. Including hand in Westminster. A full time to terrorism investigation is already under way. Hundreds of our police and security officers have been working through the night to establish everything possible about this attack. Including its preparation. Motivation. But whether there were any associates involved in its plan. And while the remain limits on what I can say at this stage I can confirm the day of the night the police have searched six addresses. I made a two rests in Birmingham and London. Mr. speaker which is still believed that this attack had acted alone. And the police have no reason to believe Barack imminent further attacks on the public. His identity is known to the peace and a lifetime. And went operational considerations about you'll be publicly identified. What I can confirm. Is that the man was British born. And perhaps some years ago and he was once investigated by MI following in relation to concerns about violent extremism. He was a peripheral pick up. The case is historic. He was not talks but the current intelligence picture. There was no prior intelligence of his intent or the cult. Intensive investigations continued. I just acting deputy commissioner readily confirmed last night. Working assumption. Is that the attacker was in response by Islamist ideology.
This transcript has been automatically generated and may not be 100% accurate.