Sources told 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.
AIR7 HD was over the facility Wednesday and 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.
An employee who works at the facility who wished to remain anonymous was shocked to hear how this all unfolded.
"It's just mind blowing that you would never suspect it," he said. "$30 million in the Valley, gone. How? Why? I'm still trying to process it. Was it an inside job? Was it just one person? Was it a group? You know, there's a lot of questions."
Employee at money storage facility reacts to massive heist: 'I'm still trying to process it'
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 thieves haven't been caught.
ABC News and the Associated Press contributed to this report.