With the deadly South Park construction fire investigation ongoing, officials say the origin appears to be an area where a spray-foam insulation trailer was being used.

According to family and their supervisor, two men who were trapped on the sixth floor of the structure Thursday were killed after initially being reported missing.

The five-alarm fire, which burned uncontrolled and threatened to spread to nearby structures for a couple hours, began around 9 a.m. The apartment building under construction, rising above two floors of garage, eventually collapsed.

Charlotte Fire Department Chief Reginald Johnson said the fire started on the ground floor of the multi-story apartment building under construction.

The fire, Johnson said, was an accident.

As The Charlotte Observer reported Thursday, a construction crew member’s 911 call reporting a trailer had caught fire inside the garage summoned the first firefighters, according to emergency radio traffic obtained by The Charlotte Observer via Broadcastify.

“I’ve never ever seen a fire of this magnitude,” Charlotte-Mecklenburg Police Chief Johnny Jennings said at a Friday news conference.

The cause of the initial spark is unclear and fire investigators are still at the scene, Johnson said.

The blaze was able to spread so rapidly because of a lack of finished walls in the building, and the breeze that carried the unrestricted flames and ash, Johnson said.

The state’s Occupational Safety and Health officers are investigating the accident. This type of investigation could take up to six months to complete said Erin Wilson, a spokesperson for the North Carolina Department of Labor.