SPOKANE, Wash. -

Investigators have determined the cause of a three-alarm fire that destroyed a Spokane medical building back in July 5th.

Char patterns on what's left of the building show the blaze was sparked by an external air conditioning unit that sat on the west side of the building.

"It originated in the area of the air conditioners. The fire investigators along with the insurance companies are going through the process of forensically examining the pieces of equipment that had electricity to them at the time of the fire," said assistant chief Brian Schaeffer with the Spokane fire department.

One of the air conditioners had been vandalized three weeks before the fire and had to be repaired by a service technician. It's possible the damage was caused by someone trying to steal the copper coils inside.

"They can make good money," said Glen Ahlborn with Action Recycling. "I mean, three dollars a pound. You figure ten pounds, which is a small amount of copper to make ten pounds, and that's $30."

The air conditioners were taped off as evidence by investigators who also looked at KXLY news video of the fire to better understand how it evolved.

Insurance companies with clients in the building will now take the cause of fire and decide whether to assign blame to the air conditioner's owner or the company that repaired it.