A former violent criminal who converted to Islam and kept out of trouble for more than a decade, the killer who struck Britain’s parliament last week was probably a “lone wolf”, self-radicalized by material on the internet, investigators say.

Police say Khalid Masood, who tried to put his troubled past behind him by turning to religion, had copied the low cost, low tech attacks espoused by Islamic State. But investigators have found nothing yet to link him to extremist groups at home or militants abroad.

“The biggest question is: why did this man become a killer,” said a European source with knowledge of the investigation. “And that is the most difficult question to answer because radicalization is so complex and nuanced.

“One serious line of inquiry is that he did what he did because of something he saw on the internet.”

The day after last Wednesday’s attack by parliament, Prime Minister Theresa May said the 52-year-old had come to the attention of spy agency MI5 as a peripheral figure in an investigation into violent extremists, sparking concern the authorities should have known he was a potential threat.

But sources familiar with the inquiry have rejected those fears. They told Reuters he had appeared on MI5’s radar when they were looking into “multiple plots” in Luton, a town 35 miles (55 km) north of London, where he was living about five years ago.

Masood’s name came up in a probe into a network suspected of helping people travel from Britain to jihadi groups in Afghanistan and Pakistan, a European government source said, but nothing they have found ties him to any group, faction or known radical preachers.

Instead, investigators suspect it was reading and watching extremist material online that led him to plow a rented Hyundai car into pedestrians on London’s Westminster Bridge last week killing three people, before charging into the walled grounds of parliament and stabbing a policeman to death.

He was shot dead after an attack that lasted 82 seconds.

“I have no evidence he discussed this with others. Whilst I have found no evidence of an association with IS or AQ, there is clearly an interest in Jihad,” said Neil Basu, Senior National Coordinator for UK Counter Terrorism Policing, referring to Islamic State and al Qaeda.

The police investigation also strongly points to Masood being a so-called “lone wolf”, carrying out the kind of attack Islamic State spokesman Abu Mohammed al Adnani called for when the group was at the peak of its power in late 2014.

Since last Wednesday, police have arrested 12 people close to Masood in connection with the attack. Just one remains in custody and all but one of the others have been told they face no further action.

However, media reports say Masood accessed the encrypted messaging service WhatsApp moments before the attack, leaving open the possibility someone else could have been involved.

“Whilst we believe at this stage Masood acted alone in his execution of the attack, our investigation continues to establish whether there are any others involved in any way and I do emphasize this is a live investigation,” Craig Mackey, acting head of London police, said on Wednesday.

When and how he became he became radicalized is now the main focus of the investigation. The authorities say stints he spent in prison are probably not the answer, since he was last released more than a decade ago.

Senior counter-terrorism figures have warned in the past that some people had become radicalized in just weeks via material they accessed over the internet, and said IS and others deliberately targeted those they saw as vulnerable.

They point to cases such as that of Roshonara Choudhry who went from a high-flying university student to stabbing a British lawmaker in 2010 just months after she started listening to online radical sermons by Anwar al-Awlaki, a Web-savvy U.S.-born Yemeni al Qaeda preacher who was later killed in a drone strike.

Farasat Latif, a former director of an English language school in Luton where Masood taught, said he had shown no inclination toward violent jihadism at the time he would have been on MI5’s radar in about 2012.

“He was more apolitical than any Muslim I’ve known. There was no interest at all,” Latif told Reuters . “I just remember him being polite, pleasant, friendly, and inquisitive about Islam.”

Latif said he has encountered radicals before: he knew Taymour Abdulwahab, a Swedish national who briefly lived and worshipped at Latif’s mosque in Luton before carrying out a suicide bombing in Stockholm in 2010.

“I have come across a number of people who have gone on to do some stupid things, and with some you’re not surprised to a certain extent,” he said. “When I knew (Masood), he wasn’t a violent radical.”

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Recollections of schoolfriends, acquaintances and neighbors paint a picture of an intelligent man with a fierce temper who lived an itinerant life, while his jail terms bear testimony to his willingness to use knives when riled.

Masood was born Adrian Elms on Christmas day 1964 to a single mother in the Dartford area of Kent to the southeast of London. He was raised Adrian Ajao, taking his step-father’s surname when his mother married, although he used both names and a number of aliases during his life.

The family lived in Tunbridge Wells, an affluent town in Kent, and schoolfriends remember him as popular, sporty and intelligent. After leaving school he was drawn into a life of crime and drugs with his first conviction coming when he was 19 in 1983 for criminal damage.

In the early 1990s, his life appeared more settled after he married businesswoman Jane Harvey and they moved to rural East Sussex, living in a smart house in the village Northiam near the south coast.

His lawyer Alexander Taylor-Camara later said Masood had moved to the area “to try and give his family and himself a better and more tranquil way of life”.

Their life together came to end in 2000 after he was jailed for two years for slashing a man across the face with a knife in a pub parking lot. One former friend, Lee Lawrence, told papers that Masood had reportedly raged on the day: “I want some blood. I want to kill someone”.

After jail he moved to the nearby seaside town of Eastbourne where those who knew him told media he became a heavy drug user and got into bodybuilding. Another knife offense saw him returned to prison in 2003.

Following his release from prison he married his second wife Farzana Isaq in 2004, but the Daily Mirror newspaper said the marriage lasted just three months until she fled their home because he was violent and controlling.

He changed his name to Khalid Masood in 2005 and between then and 2008, he spent two one-year stints in Saudi Arabia where he worked as an English teacher.

“During his time in Saudi Arabia, Khalid Masood did not appear on the security services’ radar and does not have a criminal record in the Kingdom of Saudi Arabia,” the Saudi embassy in London said.

On his return to Britain, he moved to Luton where he lived with his third wife, Rohey Hydara from Gambia, and two young children. He taught adults at the English Language Adventure School where its former director Latif recalls he was a good teacher and enjoyed a positive rapport with students and staff.

Latif said Masood had no time for the radical groups who were active in the town at the time.

“He was more focused on his family, his daughters, his wife, his career and more about the basic tenets of Islam,” he said.

“I think Islam was the thing he felt was going to help him clean up. His focus in life was to stay on the straight and narrow.”

His movements after Luton are not clear. Sometime in 2013 he moved to east London, and he returned to Saudi Arabia briefly in 2015. Neighbors said he had been living in the Winson Green area of Birmingham with his family for about seven months before they suddenly moved out last December.

Hydara returned to east London while he stayed in Birmingham. He was living in an apartment in the Edgbaston area until last week’s attack.

“I know when, where and how Masood committed his atrocities, but now I need to know why,” Basu said. “We must all accept that there is a possibility we will never understand why he did this. That understanding may have died with him.”

(Additional reporting by Guy Faulconbridge in London; Editing by Peter Graff)