A wiring malfunction inside a wall led to a stubborn fire in a University of Hawaii portable building on Sunday that caused $1.25 million in damage, a Honolulu Fire Department spokesman said.

"There’s a segment of wire that (the investigator) can see that there was a build up of heat," said fire Capt. Terry Seelig. "He can’t be specific about what caused that malfunction to the wire."

The heat led to a fire that grew in the wall, then entered the room and extended to the roof, where it smoldered in an unconventional layer of wood, corrugated metal sheeting, paper and tar.

"All that composite together on top of the roof made kind of a sandwich that hid the fire until we saw smoke coming from the eaves after we had controlled the fire within the room," Seelig said. "We discovered that the whole roof was on fire and it shortly failed after that."

Firefighters classified the fire as unintentional but don’t know the cause of the malfunction because that would require expensive lab tests.

"It’s beyond our ability to say," Seelig said. "But we do see evidence of it building up heat and radiating into the wood in that location."

Seelig gave credit to UH for providing an excavator that helped to pull away hazards and allow the investigator to determine the cause.