The suspects cut through locks on three doors at the facility on 14th and San Pedro streets.
The stolen weapons include 21 MP-5 submachine guns and 12 high-caliber handguns, including 45s. The guns had been moved to the building on Wednesday night and stored in a locked box on the first floor, but when a SWAT unit arrived the next morning, they were gone.
The weapons had been modified to fire only simulated rounds, like plastic bullets, but they can be converted back to lethal use. It would just take specialized equipment and some know-how.
As a rule, LAPD officers are not to leave weapons unattended. There's no word on whether anyone will be disciplined.
LAPD officials say they are looking into possible policy violations, while also trying to track down the stolen guns.
"We're going to get to the bottom of that, find out what the policy was with respect to these weapons," said LAPD Cmdr. Andrew Smith.