Police have appealed to anyone who knew the Westminster attacker Khalid Masood to come forward with information.
Assistant Deputy Commissioner Mark Rowley said he wanted information about Masood’s “associates and places he recently visited”.
The fourth victim has been named as 75-year-old Leslie Rhodes, from Streatham in south London – one of three people killed by a car on Westminster Bridge.
Police said two more “significant arrests” have been made.
Fifty people were injured in the attack, with 31 receiving hospital treatment. Two are in a critical condition; one has life-threatening injuries.
Updating reporters outside New Scotland Yard, Mr Rowley said two police officers remain in hospital with “very significant” injuries.
Masood, 52, drove into pedestrians on Westminster Bridge before crashing his car into railings and then running into the grounds of Parliament, armed with a knife, where he was shot dead by police.
He used a number of aliases and was known to police. Mr Rowley said his birth name was Adrian Russell Ajao.
Mr Rowley said: “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.”
Masood – who was born in Dartford, Kent – was known to the police.
His previous convictions included causing grievous bodily harm, possession of offensive weapons and public order offences.
He was believed to have been living in the West Midlands, and had previously spent time in Crawley, West Sussex, and Rye and Eastbourne, both in East Sussex.
Aysha Frade, a teacher in her 40s, and US tourist Kurt Cochran, 54, were killed on Wednesday, while Mr Rhodes died on Thursday evening when his life support machine was turned off.
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