Raids have taken place across the UK in the wake of Wednesday's terror attack.
The police have focused on a number of places that have previously been associated with extremism.
According to the Metropolitan Police, Masood lived most recently in Birmingham where he hired a car to carry out the Westminster terror attacks.
A recent study found that a tenth of all Britain’s convicted Islamist terrorists - 26 in total - came from just five council wards in Birmingham.
The wards — Springfield, Sparkbrook, Hodge Hill, Washwood Heath and Bordesley Green — contain sizeable areas where the vast majority of the population is Muslim.
The city as a whole had 39 convicted terrorists, more than West Yorkshire, Greater Manchester and Lancashire put together.
Birmingham was at the centre of the “Trojan Horse” scandal, when Islamists tried to take over state schools.
Sir Michael Wilshaw, the former head of Ofsted, said that the “second division” performance of the cities schools presented “real dangers that radicalisation will arise again and regressive forces will come back”.
According to electoral records Masood lived in Luton at two addresses between 2011 and 2013.
Almost a quarter of the population of Luton is Muslim and the town has been described as a “hotbed” of terrorism.
Last month a group of extremists with links to the hate preacher Anjem Choudary were jailed for praising Isil and urging people to travel to Syria to fight.
The banned extremist Muslim group al-Muhajiroun was based there and it is where the July 7 bombers met before they launched their attacks on London's transport network in 2005.
In 2010 an Islamic fundamentalist who carried out a suicide bombing on a busy street in Sweden was radicalised in Luton.
Junead Khan, a delivery driver from Luton, was convicted last year of plotting to behead a US serviceman in Britain outside RAF Mildenhall in Suffolk.
His trial heard that he may have been planning to carry out an attack on Westminster. His trial revealed that he had also posted a video on Youtube called 'ISIS drives around Westminster', which featured music praising ISIS.
Forest Gate, East London
In 2014 Masood lived in Forest Gate, an area which has been repeatedly linked to terrorism.
There have recently been a string of arrests for terror-related offences and a third of the population is Muslim.
The area remains most notorious for a bungled raid in 2006, when counter-terrorism officers wrongly arrested two brothers and shot one of them.
But no device was found and the men were released without charge after spending a week in custody. Police later agreed to pay the brothers £60,000 in damages.