A former Marine who lives in Washington was arrested by FBI agents in the District Thursday evening after he tried to purchase an AK-47 from an informant he believed was a part of the Islamic State.

The FBI began investigating 37-year-old Clark Calloway last June after he posted items on Facebook that included “pictures associated with jihad and terrorism, including individuals walking with rocket launchers, machine guns and other weapons, and individuals carrying the black flag commonly associated with [ISIS],” court documents state.