The cause of a massive fire at an apartment complex under construction in Sugar House last fall, which prompted hundreds of tenants to be evacuated from an adjacent complex for days and some businesses to close for two weeks, is officially considered to be "undetermined."

That's according to a final report on the incident.

However, the most likely culprits are either large space heaters that were placed on the upper floors just days prior to the fire igniting, or an electrical problem, according to investigators from the Salt Lake City Fire Department and the Bureau of Alcohol, Tobacco, Firearms and Explosives.

"With not being able to fully rule out the heaters or the electrical system, the actual cause of the fire cannot be determined with the available data at the time of this report. Therefore, the classification of the fire is undetermined," the report states.

The large fire was first reported about 11 p.m. on Oct. 25 at an eight-story residential and commercial structure under construction next to the occupied Sugarmont Apartments. Hundreds of residents were forced to evacuate from nearby apartments. Many were unable to return to their homes for a few days as crews continued to put out hot spots and evaluate the safety of the structures. Some businesses were unable to reopen for two weeks.

The fire impacted the Residences at Sugar Alley, 2188 S. Highland Drive, the Sugarmont Apartments and Townhomes, 2191 S. McClelland Street, and the View at Sugarhouse Crossing, 2120 S. Highland Drive.

A firefighter pours water on a blaze at an apartment building under construction near 1040 E. 2220 South in Sugar House in Salt Lake City on Oct. 26, 2022. The fire, which started the night before, prompted the overnight evacuation of hundreds of nearby residents.
A firefighter pours water on a blaze at an apartment building under construction near 1040 E. 2220 South in Sugar House in Salt Lake City on Oct. 26, 2022. The fire, which started the night before, prompted the overnight evacuation of hundreds of nearby residents. | Jeffrey D. Allred, Deseret News

Salt Lake fire investigators and the ATF spent weeks interviewing tenants and workers from Keir Construction, and sifted through what remains they could, trying to determine how the fire may have started.

KSL.com obtained a copy of their 98-page final report in a public records request.

In the report, multiple construction workers told investigators about portable heaters being brought in which made their work areas extremely hot, particularly on floors four through eight.

"The overall responses from the interviews are that there were two heaters on every upper floor," the report states.

The heaters were supplied by gas lines.

"The heaters were installed the previous week," the report continued. "The heaters were placed next to the walls in the corridor. The workers stated that it was extremely hot on the upper floors and some workers even tried to block the heat by placing gypsum board in front of doorways to reduce the amount of heat going into the units they were working on."

Some workers also reported seeing construction debris, including "bags of insulation in the hallway where the heaters are located. Some workers reported the gas supply lines were in front of some of the heaters," the report states.

Many construction workers interviewed echoed the opinion of one worker who "stated the heaters were a source of discomfort as they made the work space uncomfortably hot," but workers were told not to move them, according to the report.

But fire investigators also noted that "spider boxes" had been installed on each level to supply power "for recharging of batteries for power tools as well as powering an electrical light string to provide lights in the structure."

According to the final report, the "area of origin of the fire was in the west corridor north of the firewall, on the sixth floor."

Investigators say the portable heaters and the spider boxes provided them with two theories of how the fire was ignited.

The first possibility is that the heaters "acted as the heat source and heated up the nearby fuel loads to the point of ignition," the report says.

"The fuels first ignited could have been construction debris such as sawdust, cut pieces of wood, paper, plastic, cardboard, or general trash that are normally found on construction sites," the report continues. "With the presence of the heaters being a heat source, the abundance of fuels, and adequate ventilation due to windows and patio doors being partially opened, the fire, once started, was able to easily spread in the structure."

The second possibility is that the spider boxes, which powered light strings in the corridors, started the fire.

"The light strings were in the corridors hung on the walls, on one side, near the ceiling. There could have been an electrical event that occurred creating enough heat to ignite nearby fuels and having adequate oxygen from the surrounding air to achieve sustained combustion," the report states. "The location of the spider box being approximately 50 feet from the area of origin it is more likely that it would have been a malfunction with the light strings."

Investigators estimate the total damage caused by the fire is $59 million.