Largest money heist in LA history: How did thieves pull it off?

LOS ANGELES - It sounds like something out of a movie… thieves break into a vault and steal up to $30 million in cash. 

The largest heist in Los Angeles history happened Easter Sunday at a money storage facility in the San Fernando Valley neighborhood of Sylmar. 

But how did the crooks come and go undetected? 

Stacy Porter, a retired Homeland security agent, said businesses often neglect the vulnerability of the roof, but even if people manage to make their way into the building, there should be plenty of security.

"Okay, I gained access into your facility from the rooftop. How was I able to access that vault with all that money? Did you know someone? Did someone on the inside know how to give that access code or that security code to someone to open it up," Porter said. 

PREVIOUS COVERAGE: Burglary crew allegedly nets up to $30M in one of the largest cash heists in LA history

He's not the first to speculate that it could have been an inside job.

"A place like this has systems that are very sophisticated to protect the facility. There's cameras galore, very sensitive alarms that pick up movement, sensors that pick up movement. And to finally get access to the safe, you need an access code, a passcode to get in there. Who is going to know that?" retired LAPD detective Moses Castillo told FOX 11.

It's believed the heist started late Saturday night and ended around 3:45 a.m. Easter Sunday. The building was empty due to the holiday. 

According to a source, the burglars cut through the concrete of the building. They cut a hole in the ceiling to get to the top of the vault. They then cut through the bolts of the vault. The source said they left behind a ladder and several tools. 

The business reportedly didn't know about the crime until employees opened the vault on Monday.  According to reports, the alarm did go off and LAPD responded but didn't find anyone, so they left. 

There is apparently security video that shows at least four people leaving from a hole, and its reported that others served as a lookout. 

The company is still auditing, but it's possible the thieves took less than $30 million.

Gardaworld is a cash management company that picks up cash bags from various businesses, ATMs and armored trucks. 

The FBI is now investigating the case along with the Los Angeles Police Department.