The threat from Islamist terrorism is "very real," but the public should not – and would not – be intimidated, U.K. Prime Minister Theresa May told the House of Commons on Thursday.
"We are not afraid and our resolve will never waver in the face of terrorism," May said.
The man responsible for Wednesday's terrorist attack on the British Houses of Parliament was a "peripheral figure" known to MI5, May said.
The British-born man, who plowed a car into pedestrians on London's Westminster Bridge, killing three and seriously injuring several others, was previously investigated by intelligence services for extremist behavior but was "not part of the current intelligence picture," May said in an announcement to the House of Commons Thursday morning.
The man is believed to have worked alone, however, British police have so far searched six addresses and made eight arrests in London and Birmingham.
May said that the man's identity will be disclosed when operational procedures allow.
She added that the police have no reason to believe there are further imminent threats to the public.
"The man was British-born and that some years ago he was once investigated by MI5 in relations to concerns about violent extremism," said May.
"He was a peripheral figure. The case is historic. He was not part of the current intelligence picture."
May added that there was no prior evidence of the plot or the perpetrator's intent.
Further investigations are ongoing.
Four people including the assailant are among the dead. A further 40 people were injured in Wednesday's attack, seven critically.
The injured include 12 Britons, three French children, two Romanians, four South Koreans, one German, one Pole, one Irish, one Chinese, one Italian, one American and two Greeks.
May confirmed that the attack would not delay Britain's plans to trigger Brexit talks on March 29.