The Los Angeles County Sheriff's Department have found more than 500 guns inside the home of a convicted felon after a tipoff from a neighbour.
Deputies arrested Manuel Fernandez, 60, after a two-day search of his home revealed the massive illegal arsenal.
He was charged with being a felon in possession of a firearm, and illegally having an assault rifle and large-capacity magazines.
He was released on a $35,000 (£27,000) bond one day after his arrest.
Authorities say their Palmdale station received a tip that a man in the Agua Dulce region - about 45 miles (75km) north of Los Angeles - was in possession of a large amount of guns despite having a criminal record.
A search on Thursday uncovered 432 firearms, the sheriff's department said in a news release on Monday.
Authorities returned the following day to find "an additional 91 firearms that were hidden within the home", and to seize computers and other electronics that were used to illegally purchase the weapons.
The investigation led detectives to the home of another woman, where police found a further 30 guns.
The woman was not home at the time and has not returned since.
"This case is a testament to the community's involvement in reducing crime and taking guns out of the hands of criminals," said Los Angeles County Sheriff Jim McDonnell.
Americans who are convicted of a serious crime, known as a felony, lose their legal right to own a firearm.
According to the LA Times, a source familiar with the investigation says the owner appears to be a firearms collector rather than someone with plans to use the guns for violence.
Agents with the federal Bureau of Alcohol, Tobacco, and Firearms (ATF) are examining the weapons - some appear to be decades old - in an attempt to trace their origin.
Investigators are currently preparing the case against Mr Fernandez for prosecution.