So many Islamist militants have been born in Birmingham — or have passed through — that the Birmingham Mail newspaper once lamented that the city had the dubious distinction of “Terror Central.”

“The extremist schools of thought seem to have become more embedded in Birmingham than in other parts of the country,” said Nazir Afzal, the former chief crown prosecutor for northwest England, who is from Birmingham.

Mr. Masood, who converted to Islam in his late 30s, was born and raised in an affluent village in southeast England. He spent much of his adulthood in and around London, interrupted by jail time and two yearlong relocations to Saudi Arabia. But Birmingham was his last residence.

Birmingham was the birthplace of Britain’s first suicide bomber, the residence of a financier of the Sept. 11 attacks, and the place where Al Qaeda hatched a plot to blow up a commercial airliner in 2006. When a masked member of the Shabab, the Somali extremist group, celebrated the murder of the soldier Lee Rigby in a 2013 video, he listed Birmingham as the first source of its fighters.

The man who is believed to have recruited the militant known as Jihadi John, the Islamic State executioner with the King’s English accent, was from Birmingham, as was his closest associate. Other prominent militants who have come through the city’s underground networks include Abdelhamid Abaaoud, organizer of the 2015 Paris attacks, and Mohamed Abrini, a Belgian national who helped plot the 2016 Brussels attacks.

In 2014, Birmingham was at the center of a so-called Trojan Horse plot in which, it was alleged, a group of Islamist extremists had sought to infiltrate and take over two dozen state schools. A recent report by the Henry Jackson Society, a politically conservative research organization, found that one in 10 convicted Islamist militants in Britain come from five Birmingham neighborhoods.

David Videcette, a former senior counterterrorism official, said that Birmingham had a better established extremist network than London — a city of seven million — which helped to explain why, in his view, many investigations lead “back to Birmingham.”