It has been pointed out, in a comment below, that this was an Ahmadi mosque and Ahamadis (often wrongly considered to be the "good Muslims") are considered heretical by most other Muslims. The attack may therefore have been carried out by Muslims. No doubt, the predictable Establishment denunciations of "Islamophobia" would be seen as a bonus. Apparently a 14-year-old and a 16-year-old have been arrested in connection with the fire (see here). Because of their age, their names may not be released.