A British-born convert to Islam, Masood had shown up on the periphery of previous terrorism investigations that brought him to the attention of Britain's MI5 spy agency.
But the 52-year-old was not under investigation when he sped across Westminster Bridge on Wednesday, plowing down pedestrians with a hired car before running into the parliamentary grounds and fatally stabbing an unarmed policeman.
He was shot dead by police.
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 U.S. government source, who spoke to Reuters on condition of anonymity.
Britain's senior counter-terrorism police officer, Mark Rowley, told reporters: "Our investigation focuses on understanding his motivation, his operation and his associates."
Islamic State claimed responsibility for Masood's attack, although it was unclear what links - if any - he had with the militant group. Police said there had been no prior intelligence about his intent to mount an attack.
Born Adrian Russell Ajao in Kent to the southeast of London on Christmas Day in 1964, he moved though several addresses in England and lived recently in Birmingham, central England.
The Daily Mail newspaper said he was brought up by his single mother in the town of Rye on England's south coast, later converting to Islam and changing his name. Other media reports said he was a married father-of-three and a former English teacher who was into bodybuilding.
Police have released few details about Masood and what might have led him to carry out Wednesday's attack, the deadliest in Britain since four young British Islamists killed 52 people in the 2005 London suicide bombings.
Known by a number of aliases, he racked up a string of convictions, but none for terrorism-related offences. His occupation was unclear.
He first came to the attention of the authorities in November 1983 when he was found guilty of causing criminal damage. His last conviction came 14 years ago in December 2003 for possession of a knife.
"Our working assumption is that he was inspired by international terrorism," said the Met's Rowley.
But Masood's age does not fit the profile of militant attackers, who are typically younger than 30, according to counter-terrorism officers.
Shashank Joshi, Senior Research Fellow at London's Royal United Services Institute, said an MI5 report based on studies of several hundred British extremists found that half were born in the UK. Few were raised in very religious households and many, like Masood, were converts.
"Masood is not atypical in being a British-born convert with a criminal record. He was slightly more unusual in being older, but we do not know how long ago he was radicalized," Joshi told Reuters.
"If it was in prison, this would be a common pathway. Given the diversity of Islamist extremists, Masood doesn't look too unusual."
Detectives are questioning 11 people in custody, having made what they said were two more significant arrests in central and northwest England.
Iwona Romek, a former neighbor from Birmingham, told reporters: "When I saw the pictures on TV and in the papers of the man who carried out the attack, I recognized him as the man who used to live next door.
"He had a young child, who I'd think was about 5 or 6 years old. 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."
In December, she said, he suddenly moved out.
Birmingham has been one of the hotbeds for British Islamists. According to a study by the Henry Jackson think-tank earlier this month, 39 of 269 people convicted in Britain of terrorism offences from 1998 to 2015 came from the city.
There are over 213,000 Muslims in Birmingham, more than a fifth of the population, according to the 2011 census, and there has been growing concern about divisions in the diverse city.
Masood hired the car he used in Wednesday's attack in Birmingham from rental firm Enterprise, and he may have rented an apartment not far from their Enterprise offices, which were among properties raided by armed officers.
On the eve of the attack, Masood spent his last night in a budget hotel in Brighton on the south coast, according to a fellow guest who recognized him.
"Nothing in his demeanor or his looks would have given me any thoughts that would make me think he was anything but normal," said Michael Petersen.