And when firefighters arrived they found flames raging on the third floor of the structure, said LAFD spokesman Erik Scott told us.
But the real find came during the firefight, he said:
First responders, he said, discovered a "marijuana growing operation" at the location.
It all started about 8:17 p.m. in a 6-story building at 421 E. Sixth St., according to an LAFD statement. The blaze required the efforts of 100 firefighters, but they had it finished in 14 minutes, the department states:
Due to a rapid aggressive attack, the flames were confined to an occupied 1500 square-foot warehouse with a partial converted living space.
A man at the scene was arrested on suspicion of arson and "cultivating plants," the LAFD stated. A woman there was also taken into custody, allegedly because she had a warrant, according to the LAFD.
The Los Angeles Police Department didn't have any more details on the arrests for us.
It's not clear what the explosion was about, but Scott said this is now an arson investigation. No injuries were reported.
Damage was estimated at only $1,500, the department said. It's not clear if the marijuana plants went up in smoke, but looking at that dollar figure we would guess not.
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Sounds of an explosion at an old downtown warehouse prompted calls to the Los Angeles Fire Department last night.