Not a great deal is known about Masood, who was shot dead by a police firearms officer after he rammed a vehicle into pedestrians on Westminster Bridge and stabbed an unarmed police officer outside parliament.
After a string of raids and arrests in the wake of the attack, two more arrests were made last night, Rowley said.
Police have made an appeal to the public for information about the Westminster attacker Khalid Masood. So far, the police believe he acted alone when he drove a hired auto into civilians on Westminster Bridge before running into the grounds of parliament and stabbing a police officer to death.
People light candles at a vigil for the victims of Wednesday's attack, at Trafalgar Square in London, Thursday, March 23, 2017.
The auto that Khalid Masood rented for the attack was rented from an Enterprise in the Birmingham area. Police said the addresses were "linked to the investigation" of Wednesday's attack.
In all, he killed four people and left more than two dozen hospitalized in what authorities called a terrorist attack.
Although some of those he was involved with included people suspected of being keen to travel to join jihadi groups overseas, Masood "himself never did so", said a USA government source, who spoke to Reuters on condition of anonymity.
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Police believe Masood acted alone, but they also want to know if he was inspired by propaganda from extremist groups such as ISIS - which issued a claim of responsibility Thursday - or if he might also have had support or direction from other people in Britain. He has been known by a number of aliases, including Khalid Choudri/Choudari. His occupation was unclear.
The man was arrested on Sunday on suspicion of preparation of terrorist acts. Masood's last conviction, in 2003, also involved a knife attack.
Mike Crofts, a former army captain who served in Afghanistan, said he was in the courtyard outside the Houses of Parliament, then rushed toward the scene and began performing first aid.
Sabrina Hussain described how she changed her mind about taking her child, nine-year-old Skye, to his daughter's birthday party just before Christmas. There was a woman living there with him, an Asian woman. "He seemed to be quite nice, he would be taking care of his garden and the weeds", Iwona Romek, 45, told reporters at her home.
"To those individuals who have interpreted and commented on what my thoughts were in that horrific and distressful moment, I would like to say not only have I been devastated by witnessing the aftermath of a shocking and numbing terror attack", she said in a statement.
"We must all accept that there is a possibility we will never understand why he did this, "BBC quoted Deputy assistant commissioner Neil Basu as saying".
British police have released little detail about the man and what might have led him to carry out Wednesday attack.