The FBI assisted U.S. Capitol Police with their investigation into the man charged with carrying a loaded shotgun near the U.S. Supreme Court.

The federal agency was brought in to help examine explosive materials that were discovered on Feb. 8 inside the seized truck driven by the gunman.

Michael Gorbey, a convicted felon, was taken into custody on Jan. 18 and charged with felon in possession of a firearm.

Capitol Police say a “suspicious substance” was found inside the green Chevrolet pickup truck that Gorbey drove, but that it was not a bomb and that it lacked an explosive device.

A source told The Hill that the materials could be used to make a pipe bomb.

Sgt. Kim Schneider, a Capitol Police spokeswoman, said that additional charges are expected.