The Islamic State group has claimed Masood was a "soldier" carrying out its wishes for supporters to attack Western countries.
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. Daesh terrorist group has claimed responsibility for the attack.
Basu added that the police still need "to establish with absolute clarity why he did these unspeakable acts", stressing that the reasons behind the attack "may have died with him [the attacker]".
"Whilst the attack lasted only 82 seconds it will remain in the memories of many forever", said Deputy Assistant Commissioner Neil Basu, who is the Senior National Coordinator for UK Counter Terrorism Policing.
The Daily Mail newspaper said Masood had been born Adrian Elms and was brought up by his single mother in the seaside town of Rye on England's south coast, later converting to Islam and changing his name.
The picture shows bearded terrorist Khalid Masood, formally known as Adrian Ajao.
British-born Islamic convert Masood, 52, was shot dead on Wednesday after killing four people including a policeman in a rampage on Wednesday when he rammed his auto into pedestrians and tried to force his way into the parliament building in central London.
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Although 11 people were arrested in the aftermath of the attack, nine have since been released, with no further action to be taken.
Officers have seized 2,700 items, including "massive amounts" of computer data, and have had contact with 3,500 witnesses to the attack, many of them of different nationalities, Rowley said. London's top anti-terror officer says two more "significant arrests" have been made in connection with the Westminster attack, in central and northern England.
There are fears Khalid Masood may have been encouraged by others to carry out the attack, as three more people were today arrested and police probe the theory he had been radicalised before striking in London.
"His first conviction was in November 1983 for criminal damage and his last conviction was in December 2003 for possession of a knife".
"Our working assumption is that he was inspired by worldwide terrorism", said Britain's most senior counterterrorism police officer, Mark Rowley, adding: "Islamist-related terrorism is our assumption".
A police officer stand outside an address in Birmingham, where cops conducted raids early Thursday..
A woman aged 32 was also held in Manchester this morning.