The LAPD uncovered a massive, makeshift armory in the ritzy Los Angeles community of Holmby Hills.
A spokeswoman for the ATF says authorities received an anonymous tip about a person selling firearms illegally.
They found more than 1,000 pistols, shotguns and assault rifles of all calibers, makes and models laid out on the floor and ammunition was piled up in several rooms.
Tools and equipment to manufacture firearms are also found inside the residence.
LA police say they have never seen such a big arsenal in one residence.
The LAPD arrested one-man and charged him with illegally making and selling of assault rifles.
Police did not release any more information about the arrest but they did say the investigation is expected to be lengthy.