The FBI and Department of Homeland Security have sent out a bulletin to law enforcement officials warning that U.S. airstrikes in Iraq could provoke retaliation by Islamic State sympathizers in the U.S., Fox News has learned.

The warning on Friday comes shortly after U.S. intelligence officials said there is evidence foreign fighters have returned to their home countries and started Islamic State cells. While there is no evidence of a credible plot against the U.S. now, the latest bulletin warns that the lone wolf nature of the threat would make it difficult to thwart.

“It is difficult to predict triggers that will contribute to [homegrown violent extremists] attempting acts of violence,” the bulletin said. “[Lone wolf terrorists] present law enforcement with limited opportunities to detect and disrupt plots, which frequently involve simple plotting against targets of opportunity.”

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