UPDATED | December 11, 2014 9:35 PM

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DALLAS (CBSDFW.COM) – Three workers, who were unaccounted for in the hours after a fire at downtown Dallas’ Thanksgiving Tower, have been found dead inside the high-rise. Their bodies were found in a thermal tank at the bottom of a 35-foot shaft on a lower parking garage level.

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Firefighters were unable to initially get to the workers, even after the fire was out, because of thick smoke, low visibility and heat.

Thursday evening the Dallas County Medical Examiner’s Office confirmed that Luis Carrillo, 43, and Oscar Esparza-Romo, 36, were two of the three people killed.

Firefighters responded to the building, located at 1601 Elm Street, at 10:18 Thursday morning. Smoke could be seen coming from the parking garage, and initially firefighters thought they were dealing with a car fire. Firefighters later determined the source of the fire coming from the area where the workers bodies were later found. At 5 p.m., firefighters were still trying to remove the bodies from the building.

Investigators have not yet determined a cause of the fire. Jason Evans, the public information officer for Dallas Fire Rescue, said, “Our investigators are in the process of looking into everything. [They’re] getting statements and looking at evidence to come up with a definitive cause.”

The workers were working on a storage tank, which is a part of the building’s HVAC system.  The workers were inside the tank when the fire occurred.  They were subcontractors, employed by Best Mechanical.

“Our heartfelt sympathy goes with these families especially at this time of year, and as we know more of course we’ll reach out to those families but it was just a very, very tragic accident,” said Cheri Torres, a spokesperson for the company.

Torres also said the workers did have safety equipment and evacuation procedures in place, although she could not comment on what might have gone wrong.

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About 2800 people were evacuated from the building, many of them forced to walk down dozens of flights of stairs to escape the 50-story building. Three people were taken to the hospital for non-life threatening injuries.

Investigators and crews from Dallas Fire Rescue will be back at the scene Friday.

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