Curtis Culwell Center attack
One of the two attackers, Elton Simpson, had linked himself to ISIL on Twitter before his attack on the exhibition of drawings of the prophet Mohammed at the Curtis Culwell Center in Garland, Texas. He had been a member of a since-closed Phoenix mosque; he and his partner, 34-year-old Nadir Hamid Soofi, who had gone to college in Utah, had been known to and were investigated by law enforcement, but nevertheless, officers did not know that they were planning an imminent attack of this type, and they were not thought to be an imminent threat, despite having admired the late Anwar al-Awlaki, and swearing loyalty to Abu Bakr al-Baghdadi, prominent figures in al-Qaeda in Iraq and in ISIL, respectively.
Sources: CNN