It wasn’t the deadliest attack in Europe linked to the Islamic State group, but it was among the most disturbing: One evening in 2016, an assailant killed two police officers in their family home, in front of their 3-year-old son. The attacker, Larossi Abballa, was shot to death by police. According to court documents, he told police negotiators that he was responding to an IS leader’s call to "kill miscreants at home with their families."