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.

Calloway's Facebook also bore comments about hating white people and wanting to attack Washington's First District police headquarters.

"Soon the great killing will commence," Calloway wrote in a Facebook post April 20.

Calloway worked in construction in Washington. He pleaded guilty in 2002 to stabbing someone and served time in prison for that crime. Calloway had been ordered to receive mental health and substance abuse treatment.

He was booked on two federal charges: possession of a firearm by a felon and transportation of a firearm with the intent to commit a felony.