'Significant arrest' made in the northwest in connection with Westminster attack
Scotland Yard's top anti-terror officer Mark Rowley said the suspects were held overnight in the West Midlands and north west as he revealed that the killer's birth name was Adrian Russell Ajao.
Mr Rowley appealed to the public for information about the killer, who was also known as Adrian Elms but was using the name Khalid Masood at the time of the attack.
The officer also revealed that the fourth member of the public injured in the attack, who died in hospital last night, was 75-year-old Leslie Rhodes from Streatham, south London.
Mr Rowley said two people remain in hospital in a critical condition, one with a life-threatening injuries. Two police officers injured in the attack are also in hospital with "significant injuries".
Mr Rowley said: "We remain keen to hear from anyone who knew Khalid Masood well, understands who his associates were and can provide us with information about places he has recently visited.
"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."