“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.

Rowley said police had made two further “significant” arrests overnight, one in the West Midlands and one in the north west of the country.

“We now have nine people remaining in custody, and one woman has been released on bail,” he said.

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.

Rowley said police carried out more than dozen searches, seizing 2,700 items including “massive amounts of computer data” and were attempting to work out whether others had “encouraged, supported or directed him”.

The Islamic State group claimed responsibility, saying the attacker was one of its “soldiers”.

“There might be people out there who did have concerns about Masood but did not feel comfortable for whatever reason in passing those concerns to us,” Rowley said, appealing for them to come forward.

The anti-terror police chief said “at least 50” people were injured when Masood ploughed through pedestrians on Westminster Bridge before knifing a policeman to death just inside the gates of Britain’s parliament and being shot dead by another officer.

A total of 31 people of 12 nationalities have been treated in hospital and one person has “life-threatening injuries”, Rowley said.

He also named a 75-year-old man who died of his injuries in hospital on Thursday as Leslie Rhodes from Streatham in south London.

His death brought the number of victims to four.

Additional reporting by Agence France-Presse