Los Angeles police and the FBI are investigating a daring heist of nearly US$30 million in cash after burglars apparently broke through the roof of a money storage facility and cracked a safe.

David Cuellar of the Los Angeles Police Department said the robbery – one of the largest cash thefts in the city's history – took place on Easter Sunday in the northern suburb of Sylmar.

The Los Angeles Times, citing sources familiar with the investigation, said burglars busted through a roof to access the building's vault, somehow evading a sophisticated alarm system.

The elaborate and complex theft appeared to be the work of an experienced crew, and was only discovered on Monday when staff opened the vault, the newspaper said.

"It's just mind blowing that you would never suspect it," an anonymous employee of the facility told ABC News.

"US$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."

While the crime is reminiscent of Hollywood heist movies like "Ocean's Eleven," it follows a string of real-life, sophisticated break-ins in the region over the years.

Two years ago, thieves made off with up to US$100 million in jewels from a truck parked at a highway rest stop en route to a gem and jewellery show in Los Angeles.

Last July, a man cut a hole in the ceiling of a high-end wine store near Venice Beach, before helping himself to US$600,000 worth of fine Burgundy and Bordeaux vintages.

According to the LA Times, the largest previously known cash heist in the city's history came in 1997, when US$18.9 million was stolen from an armoured depot. The thieves were eventually caught. (AFP)