Los Angeles police and the FBI said on Thursday that they are investigating a major heist that occurred over Easter.
The heist took place on Easter Sunday in a building in the northern suburb of Sylmar, David Cuellar of the Los Angeles Police Department (LAPD) said.
Media reports said nearly $30 million (€27.68 million) in cash was stolen.
It is thought to be one of the largest burglaries in Los Angeles' history.
What are the details of the Los Angeles heist?
The regional Los Angeles Times daily reported that burglars broke through a roof to access the building's vault. It was unclear how the thieves managed to bypass the building's alarm system.
The theft was only discovered on Monday when staff opened the vault, the Los Angeles Times said.
Media reports identified the building as a facility belonging to GardaWorld, a Canada-based cash management and security company.
Aerial footage from the ABC broadcaster showed a cutout on the side of the building that appeared to be covered by a piece of plywood.
What security experts say about the heist
Security and armored car expert Jim McGuffey told the Associated Press that the heist was a "shock."
He said that any facility like the GardaWorld building in Sylmar would have two alarm systems and a seismic motion detector on the safe, as well as additional motion sensors throughout the building.
"For that kind of money, you don't just walk in and walk out with it," he said. "A facility should be protected from the top to the bottom and the sides."
He said that GardaWorld has a good reputation in the industry, but every such company has "isolated incidents like this that just make no sense, but it happens to the best of them."