(The Washington Post)

Police on Monday released the names of two of the three men who carried out the high-profile attack in the heart of London last week — but one of them was already known for his extremist views among his neighbors and had been reported to authorities years ago.

He had also appeared in a 2016 British television documentary called “The Jihadis Next Door.”

Khuram Shazad Butt, a 27-year-old former transit worker, was apparently a sociable man. His neighbors said he invited male friends over for barbecues, and frequented a local gym and swimming pool.

But he also displayed a dark side — enough for the neighbors to call a police hotline to warn that he was an extremist.

Butt, who was killed by police during the attack Saturday, was a British citizen who was born in Pakistan. He spoke English salted with east London slang and was a fan of the Arsenal soccer team.

He was wearing one of the team’s jerseys when he and the two other attackers killed seven people and wounded 48 in the vehicular and knife attack in central London. The other attacker named by police was Rachid Redouane.

Butt had vocalized his extremist views for years, including ranting that voting in British elections was “against Islam.”

Erica Gasparri lives in the public housing complex where Butt resided in the east London borough of Barking. The mother of three told The Washington Post that two years ago, one of her children came home and told her, “Mummy, I want to be a Muslim so I can go to heaven.”

Gasparri said, “He was trying to brainwash them.”

Neighbors said Butt would not address women outside of his family. His wife often wore a full veil. He had two children.

Gasparri said she called a police hotline and reported her belief that Butt was an extremist. “He told me, ‘I will do anything for my religion,’ ” she said, referring to Butt. Gasparri took it as a threat.

“Khuram Shazad Butt was known to the police and MI5,” police said in a statement Monday, referring to Britain’s main domestic intelligence agency. “However, there was no intelligence to suggest that this attack was being planned and the investigation had been prioritized accordingly.”

Jibril Palomba, another neighbor, said he spent time with Butt at the gym, where they spotted each other while lifting weights and boxed as sparring partners. “He was a good guy, but then he turned out to be trouble,” Palomba said.

Palomba, whose parents are Italian and Somali, is a Muslim. He said the last time he saw Butt, just three days before the attack, his neighbor seemed strange.

“He said to me, ‘Good luck,’ and I said, ‘What, where are you going?’ And he said, ‘No place.’ But then he said, ‘I’m just saying, you should be a good Muslim, so that you will go to heaven.’ And I told him, ‘I am a good Muslim. What are you talking about?’ ”

Butt was once ejected from one of the east London mosques he occasionally prayed in, after reportedly shouting “Only God is in charge” and interrupting the imam. The Jabir bin Zayd Islamic Center said Monday that the incident occurred “some years ago” but did not provide details.

In “The Jihadis Next Door,” Butt can be seen praying in a London park. 

At least one other person featured in the documentary has since joined the Islamic State militant group in Syria. Siddhartha Dhar left Britain only days after being released from prison on bail and is thought to have risen in the ranks of the Islamic State quickly after his arrival in Syria. Like Dhar, Butt is thought to have been associated with al-Muhajiroun, a banned extremist organization

Al-Muhajiroun’s longtime leader, Anjem Choudary, a cleric, has been accused of radicalizing dozens, if not hundreds, of young Britons over the past two decades. He was sentenced in 2016 to 5½  years in prison for inviting support for the Islamic State, but some experts said the sentence came too late. By that time, many of al-Muhajiroun’s associates had already gone to Syria and Iraq to join the Islamic State, and some are believed to have returned to Britain. 

“Traditionally, the British government has given those groups a lot of leeway — something which was justified with freedom of speech. But perhaps more importantly, authorities also believed that the existence of such groups would allow authorities to keep an eye on suspects, given that members were on the radar of authorities,” said Shiraz Maher, deputy director of the London-based International Center for the Study of Radicalization and Political Violence. 

“But especially people on the edges of those groups occasionally slip through and are not being monitored by authorities,” Maher said.