Sources told ABC7 Eyewitness News the theft happened at a GardaWorld facility in Sylmar.
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The thieves broke into the facility undetected - without setting off alarms - by going through the roof and somehow getting into the money storage area, which may have been a vault, according to officials briefed on the investigation.
The company that owns the building did not discover the massive theft until Monday morning.
VIDEO: Moments guards realize millions stolen in California jewelry heist of Brink's truck
VIDEO: Moments guards realize millions stolen in California jewelry heist of Brink's truck
Helicopter video Wednesday captured an apparent hole on the side of the building that was boarded up. A pile of debris was right next to it. It's unclear if that damage is related to Sunday's theft.
LAPD says the agency is working with the FBI on the case. It's unclear exactly how much was stolen, but it's believed to be one of the biggest heists in L.A. history.
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The sophisticated heist leads authorities to believe that a crew of some kind is responsible for the brazen break-in. No suspect information was immediately available.
The theft comes nearly two years after as much as $100 million in jewels and other valuables were stolen from a Brink's big rig at a Southern California truck stop. The big rig was headed from the Bay Area to Pasadena with millions of dollars worth of merchandise on board. Officials believe thieves stole diamonds and other jewels, but to this day no one has been caught and no charges filed.
ABC News and the Associated Press contributed to this report.