British police say they have released without charges six of the 10 people being held 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.
Four other people remained in custody on March 24 as authorities work to learn how Masood became radicalized before he went on his rampage. He was shot and killed by police during the attack.
Authorities have said 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.
The six people released on March 24 -- two women and four men -- were arrested in the central English city of Birmingham, where Masood had lived. They had been arrested on suspicion of preparing terrorist acts.
"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?" said Mark Rowley, Britain's top counterterrorism officer.
"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.
Aside from Masood, four people died in the attack, including a 75-year-old British man from south London who died on March 23 when he was taken off life support at King's College Hospital.
Leslie Rhodes died of injuries suffered when the assailant drove a rental car through a crowd on Westminster Bridge, which leads to the British Parliament.
The other fatalities 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.
Among the injured was a Romanian woman who fell into the Thames River when Masood's rented vehicle plowed into pedestrians during the attack.
Her country’s embassy on March 24 said she remained in serious condition but was showing signs of improvement.
Romanian officials did not identify the woman, but her friends said she was Andreea Cristea of the Black Sea port of Constanta. Her boyfriend was injured but has been released from the hospital.
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, counterterrorism officer Rowley 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 and others who traveled from as far as China to explore London.