The FBI and the Los Angeles Police Department are investigating one of the largest cash robberies in the city's history after up to US$ 30 million was stolen from a cash storage facility in San Fernando Valley.

The robbery occurred on Easter Sunday night at an unnamed facility in Sylmar, a suburban neighborhood in San Fernando Valley, where cash from businesses across the region is handled and stored, said the source.

The thieves gained access to the building and entered the vault without triggering alarms, and investigators believe it was a sophisticated group due to their ability to evade detection, said the source. One area of focus in the investigation is whether the group had insider knowledge of the facilities, said the source, who added that the robbery was discovered on Monday.

The facilities were managed by the private security company GardaWorld, according to the source. CNN reached out to the company for information on the incident.

According to the Los Angeles Times, the largest cash robbery in the city previously occurred on September 12, 1997, when US$ 18.9 million was stolen from the former headquarters of Dunbar Armored Inc. on Mateo Street. The suspects were eventually apprehended.