As they named him as the man who killed three on the streets of London, Scotland Yard revealed that Masood was a serial offender who had a string of previous convictions.
The first was in November 1983 for criminal damage.
The most recent was 14 years ago for a crime that he would commit again as he launched his deadly attack on Westminster - possession of a knife.
In between he was well-known to police with a series of convictions for assaults, including Grevious Bodily Harm, possession of offensive weapons and public order offences.
The Home Secretary Amber Rudd also confirmed that Masood had spent time in jail.
But he was also known to the security services – because of his links to violent extremists.
They had previously investigated Masood but concluded that he was on the outer circles of those that they should be concerned about.
Scotland Yard was swift to point out that Masood was never convicted of any terror offences.
He was also not subject to any investigations at the time of his murderous rampage.
Detectives have called for anyone with information about Masood to call a specialist anti-terrorist hotline.
Ms Rudd insisted that the security forces could not be held at fault and that Masood was not an individual that thy would have kept under 24-hour surveillance.
Shashank Joshi, from the security think tank the Royal United Services Institute (RUSI), said: "We have always known that it is exceptionally hard to understand who will become a terrorist.
"Masood is unusual in that only a minority become radicalised over the age of 30.
"His criminal record is unsurprising, as some studies show that a significant proportion of jihadists have had prior convictions."
He added: "Finally, it's worth noting that Masood was known to MI5. However, thousands of people will be on MI5's radar.
"Not all can be monitored indefinitely, given the limited priorities and the massive scale of the challenge.
"The most important thing is to remember that dozens of attacks like this one have been stopped in the last several years, and that no country can have a perfect record."
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