Fire crews responded to Vosburg Drive and Desert Willow Road at 1:07 a.m. Wednesday and found two houses fully involved in flames. It took firefighters about 45 minutes to knock the fire down, and one of the houses partially collapsed.
"At this point, we don't know the cause or the origin of the fire," said Azusa Police Sgt. Xavier Torres.
Torres said they are looking into a report that just before the fire erupted, an argument was heard nearby.
Arson investigators were on scene overnight, but they were not able to enter one of the houses because it was not safe. They planned to return at daylight.
The houses were part of an upscale development. The damage to the buildings was estimated at $250,000 each. If they had been completed, officials say the damage estimate would have been $750,000 each.