The four remaining in custody were arrested on suspicion of "preparation of terrorist acts", the police said. Hundreds of officers had made contact with thousands of witnesses, he said. After four days of intensive inquiries across England and Wales, involving hundreds of officers, the Metropolitan police said they had so far failed to establish the reason for the attack. Masood was also known by a number of aliases.
A 32-year-old woman arrested in Manchester was bailed pending further enquiries until a date in late March.
In Birmingham, searches are continuing at a flat raided by armed police.
According to Sky News, Masood was an English teacher and religious convert.
"We are determined to understand if Masood was a lone actor inspired by terrorist propaganda or if others have encouraged, supported or directed him", Basu said.
Rowley said that at least 50 people were injured in the attack with 31 requiring medical treatment. Two people are now in a critical condition - one with life-threatening injuries.
Among those killed was unarmed police officer Keith Palmer, whom Masood stabbed to death after slamming his rented SUV into a Parliament gate.
The fourth victim, a 75-year-old man, died Thursday night after his life support was withdrawn at a hospital where he had been receiving treatment, according to London's Metropolitan Police.
A police officer looks at floral tributes in Parliament Square, following the attack in Westminster earlier in the week, in London, Britain, March 25, 2017.
Pc Kristofer Aves was returning from a ceremony where he collected a commendation for his work as a family liaison officer when he was seriously injured as Khalid Masood drove his hire auto into pedestrians on the bridge on Wednesday.
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The attacker, who had also stayed the previous Friday, had been "very friendly, laughing and joking", and discussed his family in Birmingham, he said. His most recent conviction was in 2003 for possession of a knife.
The police have not linked Masood to a terrorist organisation.
"He was just another guest who checked into the hotel", adding that Masood discussed his family, saying his father was ill and mentioning his "wonderful children".
Masood always wore a robe and a cap and had a long beard, the neighbors said.
"We need to make sure that organizations like WhatsApp, and there are plenty of others like that, don't provide a secret place for terrorists to communicate with each other", she said.
But investigators now believe Masood was more likely inspired by ISIS rather than directed by the terrorist group, the official told CNN. The man was also detained under TACT while the woman was arrested under the Police and Criminal Evidence Act (PACE).
The 52-year-old attacker was born in southeastern England and had most recently been living in the central city of Birmingham.
"There are victims in London from 11 nations, which goes to show that an attack on London is an attack on the world", UK Foreign secretary Boris Johnson told reporters at the UN during a United States trip earlier this week.
Preston Park hotel's Sabeur Toumi revealed that "we never thought anything of him".