According to Caryn Brooks, Osman Barre Mohamud warned the authorities about his son, Mohamed Osman Mohamud, prior to his being arrested for his involvement in an a plot to bomb a Portland, Oregon, Christmas tree lighting ceremony that was set up by the FBI. This would be yet another terrorism-related arrest in which authorities were tipped off by a source in the American Muslim community. According to the FBI arrest warrant, the younger Mohamud sought ideological kinship and martyrdom not from his local mosque but from an unidentified source on the Internet.
So, of course, someone set Mohamud's mosque on fire. If you actually believe that America is at war with Islam, then all Muslims are the enemy, and there's nothing wrong with holding them collectively responsible. That is, of course, what al-Qaeda would like you to believe, since it turns them from a small, radicalized minority into a nation of billions.