Terrorism convictions in the UK have doubled over the past five years, and the number of offenders known to the authorities prior to their involvement in terrorism is a major concern, a comprehensive new study says.

The 1,000-page document analyzes aspects of all Islamist terrorist cases in Britain from 1998 to the beginning of 2017, involving 269 convictions or suicide bombings, and almost 400 offenses. The study is the biggest of its kind in Europe, according to The Sunday Times, which reviewed the study – released this week by Britain’s counter-terrorism chief, Assistant Commissioner Mark Rowley.

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Contrary to popular belief, most of the attacks included in the report were not committed by lone attackers, the report said. Only 28 of the 269 were “lone wolves,” while three quarters of those whose household status was known lived with their families.

Almost 80 percent had ties with some kind of extremist networks. In particular, al-Muhajiroun, a Salafist organization that had been banned in the UK in 2010 and again in 2014 under a new alias, has been linked to one-fourth of British terrorism convictions.

Deprivation is a major factor in radicalization, according to the study, contrary to the belief that middle-class educated youths are prone to it. Economically troubled Birmingham, with 234,000 Muslims, has a total of 39 convicted terrorists, which is more than do the better-off West Yorkshire, Greater Manchester, and Lancashire, which have a combined Muslim population of 650,000. Just four of Britain’s 9,500 council wards account for 26 convicted terrorists, or a tenth of the national total, and all four are located in Birmingham.