Britain's top anti-terrorism officer, Mark Rowley, said police had nine people in custody after the attack on Wednesday which killed five people including the assailant.
Police had said the man behind the attack was British-born Muslim covert Khalid Masood, who used several aliases, and that they were trying to establish if others had directed him.
"Our investigation focuses on understanding his motivation, his operation and his associates," Mr Rowley said.
"Whilst there is still no evidence of further threats, you'll understand our determination is to find out if either he acted totally alone, inspired perhaps by terrorist propaganda, or if others have encouraged, supported or directed him."
Mr Rowley said police had made two further "significant" arrests overnight, one in the West Midlands and one in the north west of the country.
Mr Rowley said the attacker, who ploughed down pedestrians when he sped across Westminster Bridge before fatally stabbing an unarmed policeman, had injured at least 50 people in total. Two are still in a critical condition, and one person is considered to have life-threatening injuries.
Five people were killed and about 40 injured in London on Wednesday after a car ploughed into pedestrians and a suspected Islamist-inspired attacker stabbed a policeman close to Britain's parliament.
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