CHARLOTTE, N.C. – Preliminary autopsy results on Salisbury firefighters Victor Isler and Justin Monroe show they died of heat exposure and carbon monoxide poisoning, according to the Charlotte medical examiner’s office.

Isler, 40, and Monroe, 19, were killed while battling a fire at Salisbury Millworks on Friday.

Funeral plans for Isler and Monroe were announced Monday afternoon. Isler's visitation is from 1-4 p.m. Wednesday at Lyerly Funeral Home on 515 S. Main Street in Salisbury. Monroe's visitation is from 6-9 p.m. at Miller's Ferry Fire Department. The funeral for Isler and Monroe is scheduled for Thursday at 2 p.m. at Catawba College's Omwake-Dearborn Chapel.

The cause of the fire is still under investigation. On Monday, heavy machinery lifted several tons of charred debris from the mill’s basement as a task force of more than 50 investigators searched for what started the massive blaze.

The fire was first reported at around 7 a.m. Friday at Salisbury Millworks, a cabinet-maker that was housed in an 80,000 square-foot complex just south of downtown Salisbury.

Monroe and Isler were killed when they went inside the burning facility in an attempt to stop the spread of flames.

Three other firefighters were injured.

The U.S. Bureau of Alcohol, Tobacco, Firearms, and Explosives is leading the investigation, with assistance from the N.C. State Bureau of Investigation, and several local agencies.

ATF spokesman Earl Woodham said agents were still in the preliminary stages of their investigation and were not yet able to determine if the fire was an accident or intentionally set.

Investigators appeared to focus Monday on an area that housed the company’s office and a basement storage area.