British police say they have made two more "significant" arrests in the investigation of the deadly attack near Parliament and said the birth name of the assailant, Muslim convert Khalid Masood, was Adrian Russell Ajao.
Britain's top counterterrorism officer, Mark Rowley, said on March 24 that police have nine people in custody, while a 10th person has been released on bail as the investigation continues.
Authorities have said that Masood, who had a history of violence, was thought to have carried out the Westminster attack on his own. But they have not ruled out the possibility that others may have been involved.
"Clearly a main line in our investigation is what led him to be radicalized. ... Was it through influences in our community, influences from overseas, or from online propaganda?" 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," he added.
The two arrests came overnight as a 75-year-old British man died of injuries suffered when the assailant drove a rental car through a crowd on Westminster Bridge, which leads to the British Parliament.
TIMELINE: Deadly Attacks In Europe (click to expand)
Leslie Rhodes was taken off life support at King's College Hospital, police said. His death brought the death toll in the attack to five, including Masood.
The other victims were police officer Keith Palmer, 48; Aysha Frade, a 43-year-old British woman; and Kurt Cochran, an American tourist from the state of Utah, who was in his 50s.
Rowley added that two police officers at the scene of the attack also suffered significant injuries. Two other people also remain in critical condition, one with life-threatening injuries, he said.
Masood, who was shot dead by police within the security perimeter of Parliament in the midst of the terrorist attack, also was known by several other aliases.
They said Masood had not been the subject of any current investigations by British authorities and there was "no prior intelligence about his intent to mount a terrorist attack."
British Prime Minister Theresa May told the House of Commons earlier on March 23 that the attacker was "a peripheral figure" known to security authorities and had been investigated.
Police said they were working on the assumption that Masood, who did not have any convictions on terrorism charges, was inspired by Islamist extremism. The Islamic State (IS) extremist group has claimed the assailant was one of its "soldiers."
Victims who were injured on Westminster Bridge came from 12 countries.
They included teenage schoolchildren from France, a Romanian couple, and others who traveled from as far as China to explore London. A Romanian woman was pulled alive from the River Thames after falling from Westminster Bridge during the attack.