Since the attack, police has made two more "significant" arrests overnight and a third on Friday morning.
Just one of the 11 people arrested in relation to the terror attack in London remains in police custody - the rest have been released.
According to a statement from the Metropolitan Police, Masood, aged 52, was born in Kent and detectives believe he was most recently living in the West Midlands.
Five people remain in a critical condition after Masood ploughed a auto down Westminster Bridge and stormed the Parliamentary estate armed with two blades, knifing policeman Keith Palmer.
It began at 9.33am (5.33pm Malaysian time) to signify the 933 badge number of the parliamentary protection officer who died in the attack, 48-year-old Keith Palmer.
The Islamic State group claimed responsibility for the attack.
Prime Minister Theresa May said Thursday that Masood was "investigated in relation to concerns about violent extremism" some years ago.
The latest victim died in the hospital on Thursday, bringing the death toll from his attack to four.
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There are concerns accomplices could have followed him in and killed more people.
The update comes as the Met released an image of Masood, also known as Adrian Elms and Adrian Russell Ajao, and appealed for anyone who knew him to come forward. 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. Two other people also remain in critical condition, one with life-threatening injuries.
Four people remain in police custody.
Police said their investigation into the attack would focus on Masood's "motivation, preparation and his associates".
A 21-year-old woman and a 23-year-old man who were arrested at an address in Birmingham. "I'm confident that we are doing all that we can to keep our city safe, to keep Londoners safe and to keep visitors safe", he said. The man, who has not been named, had his life support withdrawn at King's College Hospital.
A source told The Sun: "There was clear air between the spot where the terrorist was shot down and Mrs May's vehicle".
Thousands of mourners filled Trafalgar Square on Thursday for a vigil for the victims killed in a terror attack in the British capital the previous day.