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Salt Lake City fire investigators had a mystery in their hands Tuesday: What caused a portion of an under-construction apartment complex to ignite?

Fire Department spokesman Jasen Asay said the 4:37 a.m. blaze, spotted first by a passing police officer, caused about $20,000 in damage before being doused at 5 a.m.

"It was suspicious because where the fire occurred there was no power going into the building and no heaters were there, as we sometimes see in apartment buildings under construction," Asay said.

"Officially, they are calling it of 'undetermined' origin," he added, noting no clear evidence of an ignition source was initially found.

The fire started in the first floor level of the three-level complex on the southeast corner of 300 East and 500 South, which also has a subfloor parking area.

The structure itself, still largely cement, sustained no damage with the flames consuming PVC pipe, fittings and some wood pallets.

No injuries were reported.

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