DOWNTOWN LOS ANGELES — Fire burned through the roof of a one-story building in the southern Arts District this morning, but left no injuries or damage to neighboring structures. Approximately 75 firefighters knocked down the blaze at 686 S. Santa Fe in just under 40 minutes.

Fire crews responded to the greater alarm fire at 6:40am and reported it knocked down soon before 7:30am. The cause is unknown and under investigation.

KNX is reporting that the building contained a marijuana grow house, but we don't have any eyewitness reports of interesting-smelling smoke.

Reader @ch of Minor Thread sent in a set of photos from near the fire.

Update (10am): The L.A. Times now cites LAPD and LAFD as confirming that growing equipment was found:

Police and fire officials are investigating an indoor growing operation found inside a downtown L.A. commercial building that was engulfed in flames.

After the fire was extinguished by firefighters, investigators discovered growing equipment inside, said police spokesman Richard French. What was being grown though is not yet known, he said.

Fire department spokesman Erik Scott said it was a possible illegal operation.