Police are investigating a loaded assault weapon found in a parking lot a block from the Supreme Court, a law enforcement official said Friday.

Supreme Court police discovered the gun on Dec. 2 in the lot in the residential Capitol Hill neighborhood east of the court and turned it over to the Washington police, said Supreme Court spokeswoman Kathy Arberg. The U.S. Capitol is west of the Supreme Court.

Officials at the Supreme Court and Metropolitan Police Department would not identify the kind of gun found, but a senior federal law enforcement official said it was a loaded MAC-10 assault weapon. It is capable of firing many bullets over a wide area in a few seconds. The official spoke on condition of anonymity because the investigation is ongoing.

The federal Bureau of Alcohol, Tobacco, Firearms and Explosives has joined the investigation, said Officer Josue Alvida, a Washington police spokesman. No arrests have been made.

The area is heavily patrolled by police from several jurisdictions, including the U.S. Capitol Police, since many members of Congress live in the neighborhood.