The mayor of Manchester expressed shock and disbelief following the savage attack carried out on innocent attendees of an Ariana Grande concert in his city by a radical Islamic suicide bomber Monday evening.

“It is hard to believe what has happened here in the last few hours,” said Greater Manchester Mayor Andy Burnham early Tuesday.

But while a statement of shock is to be expected following such a wanton, depraved act of violence, the threat of Muslim extremist violence is becoming increasingly common in the U.K.

Western Europe, and especially the United Kingdom, has an obvious and growing problem with violent, radical Islam, and the Manchester area has one of the largest Muslim communities in Britain, a community that has produced a significant number of extremists.

Exactly two months before the attack in Manchester on Monday, a British-born radical Muslim named Khalid Masood drove a car into pedestrians on Westminster bridge before attempting to ram the vehicle through a barrier in front of the Houses of Parliament. In the following days nearly a dozen people were arrested in connection with the attack — two of whom lived in the Greater Manchester area.

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A month prior to the Westminster attack, The Times reported that Manchester-born Jamal al-Harith (born Ronald Fiddler), an ISIS fighter of Jamaican descent who at the time had recently carried out a suicide attack in Iraq, was part of an extensive terror cell from Manchester.

Harith was reportedly good friends with Raphael Hostey aka Abu Qaqa, an ISIS recruiter and Twitter propagandist who died fighting for the militant Islamic group. “Everyone knew [Harith] because he had been in Guantanamo. He came from the same area as my grandson, they all knew each other,” Hostey’s grandmother, who refused to be named, then told The Times.

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“He wasn’t famous; he was well-known in the area. My son talked about him a lot. They had been together on [an aid convoy to Gaza in 2009], there was a group of them from Manchester. They weren’t in the same van, but they traveled together.”

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Hostey’s grandmother told The Times that he and several of his friends said they were going overseas in order to study Arabic. “They went away, and the next thing was his mother’s house was raided by the police, who said they were all fighting in Syria,” she said.

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In 2014, teenage twins Zahra and Salma Halane fled their Manchester home to become ISIS brides. Salman Abedi, the perpetrator of the Manchester arena bombing, grew up in Whalley Range, the neighborhood where the Halane sisters attended school, the Daily Telegraph reported.

That is to say nothing of the numerous ISIS fighters recruited from all over the U.K. — many of whom are now returning — and the extent to which radical views are shared across the U.K. Muslim population.

In February the BBC reported that at least 850 ISIS fighters with U.K. citizenship had returned to the country, and in April of last year a poll conducted by research firm ICM and commissioned by Trevor Phillips, former chair of the U.K. Equality and Human Rights Commission, found that only a third of Muslims living in the U.K. would alert authorities if a friend or relative became involved in terrorism.

Nearly a quarter — 24 percent — said they sympathized with violence perpetrated in defense of religion.