Islamic State has claimed responsibility for the attack, but investigators have found no evidence suggesting Masood acted on orders from any group.
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.
A police officer places flowers and a photo of Pc Keith Palmer on Whitehall near the Houses of Parliament in London, after seven people were arrested in raids in London, Birmingham and elsewhere linked to the Westminster terror attack.
He had shown up on the periphery of previous terrorism investigations that brought him to the attention of Britain's MI5 spy agency.
The night before the attack, Masood stayed in a hotel in Brighton on the southern English coast, where he told staff he was visiting friends.
Police investigating the terror attack Wednesday in London think the motive behind the attack will remain unknown.
Asked about his possible conversion to Islam and subsequent radicalization, Rowley said: "Clearly that's the main line of our investigation, is what led him to be radicalized, was it through influences in a community, influences from overseas, or through online propaganda".
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"If you knew Khalid Masood and had spoken to him in the months, weeks or days leading up to this attack please come forward", said Met police Indian-origin Deputy Assistant Commissioner Neil Basu, who is the Senior National Coordinator for UK Counter Terrorism Policing. Two people are now in a critical condition - one with life-threatening injuries.
A dozen people have been arrested since 52-year-old Briton Khalid Masood launched an assault on parliament on Wednesday, killing four people and leaving 50 injured before he was shot by police. A 58-year-old man arrested in Birmingham was still in custody Saturday, police said.
The 52-year-old was born in Kent to the southeast of London and moved though several addresses in England, although he was known to have lived recently in Birmingham in central England.
A 32-year-old woman who was arrested in Manchester, remains on bail, police said.
A new breakdown of the attack revealed that Masood, a Muslim convert with a violent criminal past, took just 82 seconds to wreak havoc.
There was no prior intelligence of Masood's intent to kill and police believe he acted alone, but was inspired by Islamist-related global terrorism. He was named by Rowley as Leslie Rhodes, from Streatham in south London.
"An act of terrorism tried to silence our democracy", May told parliament.