He had at least three children.
The Metropolitan Police says he was born as Adrian Russell Ajao - but the story appears to be more confusing still, because of a string of alternative names or aliases he used.
He was entered onto the birth registry in the Dartford district of Kent as Adrian Russell Elms, in the weeks after he was born on Christmas Day 1964.
Elms was his mother's maiden name, but two years after he was born she married a man with the name Ajao.
The future killer used the surnames interchangeably before he converted to Islam and became Masood.
His mother and her husband lived for a long time in Tunbridge Wells, Kent, where the young Masood, then called Adrian, attended Huntleys School for Boys.
His parents later moved to Wales and their Carmarthenshire home was this week searched by detectives from the Wales Extremism and Counter Terrorism Unit.
They have not been treated as suspects and the BBC understands from a local source that Masood had been estranged from his parents for some time.
Masood had a string of previous convictions for violence and public order offences.
His first conviction came when he was 18, in November 1983, for criminal damage.
Ten years later he appeared to have settled down.
He and his first partner had two children and lived in the village of Northiam, East Sussex.
While living there in 2000, Adrian Elms became embroiled in a serious pub attack following what may have been racist provocation and ostracisation or arguments in his village.
He lost his temper with Piers Mott, slashing him on his face and leaving an 8cm gash on his left cheek.
Mr Mott has since died, but his widow Heather recalled the incident.
"My husband was defending someone who was working for him," she said. "I don't know how it happened - Piers was just defending this guy."
Elms was jailed for two years after admitting the attack - and never returned to live in the village, although his former partner and children remained there for some years.
After his release from jail, he moved to Eastbourne - where a man who knew him as a casual acquaintance told the BBC Elms used both cocaine and bodybuilding steroids.
He was also on the cusp of a change in identity.
"I first met him in a pub around Christmas Eve 2001," the former electrician told the BBC.
"He was introduced as 'Black Ady'. I believe he had just got out of jail at the time."
Elms came across as a Jekyll and Hyde character.
"Everyone who met him said he was really polite, but he was in a pub one night and two guys were playing pool.
"Something was said and he battered them both with pool cues.
"One time I noticed a Koran and I made a joke like: 'Are you reading that?' He did not seem the religious type at all.
"I spoke to my friend who lived with him for a while and he said he used to read the Koran all the time.
"The next time I saw him it was summer 2002 - he was on steroids, though he wasn't huge," said the friend.
"He took quite a lot of cocaine and he seemed to like women with pink hair. He had one girlfriend and she had silicone breasts."
When Elms and the acquaintance mistakenly thought they were dating the same woman - Elms suggested that if they weren't friends, he would stab him in the face.
That is exactly what he did to another man in 2003, and he was returned to prison for six months.
In total, Elms was held at three different jails - HMP Lewes, Waylands, and Ford.
That last spell in jail came when he was approaching the age of 40. In general terms, that's quite late on in life for your average angry young man stuck in a world of petty criminality, violence and robbing.
We don't know for sure when Elms converted, but in 2004 he married a Pakistani-heritage Muslim woman in Crawley, West Sussex.
At this stage his legal name was still Elms, but one of her relatives told the BBC that he was known to them as Masood.
In November of the following year, he left to work as an English teacher at the General Authority of Civil Aviation in Jeddah, Saudi Arabia.
He was registered on the electoral roll in Crawley again in 2006 - and spent a second stint in Saudi Arabia from April 2008.
This behaviour is entirely consistent with the journey of many converts to Islam: they look for opportunities to live and work in the Islamic world as part of their efforts to fully adopt the faith and way of life.
We next see Masood finally back in the UK in 2010. By then he was living in Luton - and with a different woman who has African heritage. His eldest child from his first relationship is also with them.
Six years later he was with the same woman in east London, and following the attack she was among those arrested, before being subsequently bailed. His final address was in Birmingham.
Masood hired the car used in the attack from the Spring Hill branch of Enterprise in north Birmingham earlier this week.
Within an hour of hiring the Hyundai, he is thought to have contacted the company to say he no longer needed the car.
We now know that before the attack he had stayed in a hotel in Brighton.
Preston Park Hotel manager Sabeur Toumi told the BBC that he had checked in under the name Khalid "Masoud" and paid by credit card.
He was "friendly and smiley" and explained he had come from Birmingham and was visiting friends.
The receptionist noted on their system that he was a "nice guest".
Police have now visited the hotel and removed the trouser press, linen, towels, kettle and toilet roll holder from the room - presumably to obtain DNA to verify it is definitely the same man.
On Wednesday, Masood left the hotel and later that day carried out his attack.
He has never been convicted of a terrorism offence and the prime minister told Parliament he had not been the subject of any current investigations.
However, Theresa May added that "some years ago" he was "once investigated in relation to concerns about violent extremism".
She went on: "He was a peripheral figure [in that investigation]. The case is historic - he was not part of the current intelligence picture.
"There was no prior intelligence of his intent or of the plot."
The BBC has learnt that the investigation pre-dates the height of the Syria and "Islamic State" crisis. What does it mean that he was a "peripheral figure"?
Here are just some of the possibilities:
• He was an associate or friend of a main suspect who was being monitored in some form - but turned out, at the time, to not apparently have any extremist leanings
• He could have been closer to an inner circle of aspiring extremists - but he personally was not considered to be a risk and so the operation was focused on others
• There could have been more concerning intelligence about his ideology and intent - but there was nothing that could make a criminal charge - and in time he was discounted as a serious threat
• He could, at the highest end, have been arrested in the past as part of an operation and later released without charge
The man police say was responsible for the Westminster attack has been formally identified as 52-year-old Khalid Masood.