As if to further punctuate the connection, the police announced Sunday that they had arrested an unidentified man in Birmingham as part of the investigation of Mr. Masood.
Members of Birmingham’s Muslim communities acknowledged the linkage between their city and Islamist extremism, which many attribute to poverty and drug abuse that make youths vulnerable to jihadist recruiters who operate like gangs. But Muslims in Birmingham also deeply resent what they see as a grossly unfair reputation, countering that most residents are proud and law-abiding.
Many also see their neighborhoods as reassuring refuges from the backlash of anti-Islam bigotry roiling Europe and elsewhere.
The bigotry has often focused on Birmingham. A few years ago, a Fox News terrorism commentator had to apologize for describing Birmingham as a “Muslim-only city” where non-Muslims “don’t go.”
Nonetheless, Birmingham, Britain’s second-biggest city behind London, has produced a disproportionate number of convicted Islamist militants, including some linked to the Sept. 11 attacks, and to last year’s bombings in Brussels.
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.