Sunday's killing of 16 civilians by an American soldier in Afghanistan could prompt retaliatory attacks within the United States, FBI and Department of Homeland Security officials say.

In an "awareness bulletin" sent to law enforcement agencies Wednesday, and first reported by ABC News, the agencies expressed concern "that this event could contribute to the radicalization or mobilization of homegrown violent extremists [HVEs] in the homeland, particularly against U.S.-based military targets which HVEs have historically considered legitimate targets for retaliation in response to past alleged U.S. military actions against civilians overseas."

An attack is possible, but unlikely, the bulletin said. More certain is the likelihood that the killings "will likely be incorporated into violent extremist propaganda and could contribute to an individual's radicalization to violence."

Threats may be most likely "against U.S. military targets in the homeland," an unnamed official told CBS. In December, an investigative report from the House Homeland Security Committee's Republican staff said that the Pentagon considers the U.S. homeland to be "the most dangerous place for a G.I. outside of foreign warzones," and the top threat faced by American soldiers at home "is from violent Islamist extremists."

Court records show that threat is fueled by a false belief that America's war against terror is a war on Muslims.