According to a Joint Intelligence bulletin from the FBI and the Department of Homeland Security sent to U.S. law enforcement officials, an ISIS spokesman recorded an audio message that urged lone offenders in Western countries to attack "soldiers, patrons, and troops … their police, security and intelligence members." Attackers did not need to "ask for anyone's advice" prior to striking, said the message, because such actions are legitimate, reports NBC News.

An English language translation of the message, which was attributed to ISIS spokesman Abu Muhammad al-Adnani, was posted on a jihadi forum in late September.

Also in September, according to the bulletin, an ISIS supporter posted a document in an ISIS-dominated web forum advocating "open source jihad" and "lone wolf operations" by U.S. Muslims against "a list of potential targets, including military, law enforcement, FBI personnel, government officials and media figures."