When it was revealed that the Boston Marathon bombers attended a Cambridge, Mass., mosque, its leaders were quick to disavow their actions. Elder brother Tamerlan Tsarnaev’s ideology was not their own, the leaders of the Islamic Society mosque claimed. In fact, he was admonished for an extremist outburst he made during one sermon. So, one crackpot in a congregation. Who can blame the mosque? But what about eight — including a prominent member of ISIS? As it turns out, worshippers at the Islamic Society have included: Abdurahman Alamoudi, the mosque’s founder and first president who in 2004 was sentenced to...