HOUSTON -

Five CenterPoint Energy employees were hospitalized after a flash fire Thursday morning at a downtown Houston building. The fire was quickly contained, but the five workers suffered what are described as critical electrical burns.

It all happened in an underground electrical vault that's located under a sidewalk. The vaults hold electrical transformers and equipment in the downtown area.

A spokesperson for CenterPoint said the employees were doing routine maintenance on CenterPoint equipment at the time of the fire. Workers could be seen blocking off the area around an underground electrical vault shortly after a flash fire that injured five workers.

Robert Reyes was nearby when he heard what sounded like an explosion and then saw the fire coming from the vault.

"It was flames. It was flames pretty high  and then I saw the flames and people coming out," said Reyes.

According to Reyes, one of the men was on fire as he came up to street level.

"He just went on the ground and started rolling to put himself out," he said.

The Houston Fire Department said some of the workers had be rescued because they were unable to get out of the vault on their own.

"Their injuries are significant. I would say some are critical," said HFD Captain Ruy Lozano.

He said the walls of the vault were built to prevent fire from spreading to the nearby Southwestern Bell building.

Still, the extreme amount of electricity caused critical burns to the five workers inside.

"That vault will carry up to 480 volts. And when you talk about a flash of that magnitude, you're expecting significant injuries," Capt. Lozano said.

For the rest of the day, workers could be seen around the site of the fire. A spokesperson for the company said an investigation will be conducted to determine what caused the flash fire.

The five employees were taken to Memorial Hermann Hospital. At this point, the extent of their injuries is unknown.

CenterPoint spokeswoman Alicia Dixon said the building and two other structures were impacted -- losing electricity because of the accident or power was later halted to make repairs.

The basement suffered minimal fire damage.

CenterPoint officials said it will be conducting an investigation into what happened.