PHOTO: Police released an image of Masood as a photo from his school days in Tunbridge Wells emerged
Police have appealed for information to trace any "associates" of the Westminster attacker, Khalid Masood.
They have named his fourth victim as retired window cleaner Leslie Rhodes, 75, from Clapham, south London - one of three people killed by Masood's car on Westminster Bridge on Wednesday.
A Romanian woman who fell from the bridge is stable in hospital.
Police said they had made two more "significant arrests", taking the total number to 10.
Nine people are still being questioned by police.
Fifty people were injured in the attack, with 31 receiving hospital treatment. Two are in a critical condition; one has life-threatening injuries.
The injured include Andreea Cristea, who had an operation for a blood clot on her brain after Masood's car hit her on the bridge, pushing her into the river.
Assistant Deputy Commissioner Mark Rowley said two police officers remained in hospital with "very significant" injuries.
PC Keith Palmer died after being stabbed in the attack.
Masood, 52, mounted the pavement and drove into pedestrians on Westminster Bridge before crashing his car into railings and running into the grounds of Parliament.
Armed with a knife, he stabbed PC Palmer, before being shot dead by police.
The Metropolitan Police said Masood had used a number of aliases and had been known to police.
His previous convictions included possession of offensive weapons and public order offences.
Born as Adrian Russell Elms, in Dartford, Kent, Masood was also known as Adrian Russell Ajao in childhood after taking his stepfather's name.