The 52-year-old terror suspect who rammed a high-speed auto into the side railings of the House of Commons before stabbing police officer Palmer to death at the entrance of the Houses of Parliament is said to have used a number of aliases, including Adrian Elms, and was known to United Kingdom police.
Rowley said police carried out more than dozen searches, seizing 2,700 items including "massive amounts of computer data" and were attempting to work out whether others had "encouraged, supported or directed him". Two of the at least 50 people injured were described as "critical" and one person had "life-threatening injuries".
The death toll in Wednesday's terrorist attack near the U.K. Parliament has risen, after a 75-year-old man died of his injuries.
Arrests were made in London and Birmingham, while homes were also searched in Brighton and Carmarthenshire, in west Wales.
The investigation has reportedly revealed the middle-aged man had many aliases, and it's been confirmed he had a string of previous convictions after he was unmasked as the culprit behind Wednesday's terror attack.
Another person, a woman, was arrested and released on bail. Nine were detained under the Terrorism Act and one was detained under the Police and Criminal Evidence Act.
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Basu said that even if he acted alone, police need to give as much of an explanation as possible to reassure Londoners.
Another man, 58, was also arrested Friday morning at another address in Birmingham, and held on the same charge.
Police are investigating whether Masood, who killed four people outside Britain's Parliament, used the encrypted messaging service WhatsApp moments before driving across Westminster Bridge on high speed mowing down pedestrians on Wednesday.
The attacker was born in southeastern England and had most recently been living in the central city of Birmingham. His last conviction was in December 2003 for possession of a knife. British Prime Minister Theresa May said Masood was not part of the "current intelligence picture".
Islamic State claimed responsibility for the attack and police inquiries since have concentrated on whether Masood was acting with accomplices. It is unclear when he converted to Islam (his mom was Christian), though there are reports he traveled to Saudi Arabia for year-long stays in November 2005 and April 2008; the BBC says that the embassy's records indicate he went there to teach English.
They also appealed for information about the homegrown killer who left four people dead. The police identified him as Leslie Rhodes from South London.
Rowley said that despite the attempt, terrorists targeting London would not succeed.