HALLANDALE BEACH, Fla. -- Hallandale Beach Fire Rescue is investigating a fuel tank explosion that happened at the U-Gas station at 990 West Hallandale Beach Boulevard Sunday afternoon.

Employees in nearby businesses said they smelled putrid fumes moments before the incident.

"We just heard a lot of chaos," said a worker at a nearby Metro PCS store.

"Units arrived six minutes later to find that there had in fact been an explosion," said Division Chief Richard Stover with Hallandale Beach Fire Rescue.

Four men were working underground. Two were inside a fuel tank.

"The tanks were being upgraded to meet EPA standards," Stover said.

Crews were were re-lining the insides to prevent leaking.

But something went wrong. Something sparked. Firefighters said it was a flash fire that scorched the two workers inside.

They were taken to Memorial Regional Hospital in Hollywood with second and third-degree burns on 70 percent of their bodies.

For almost three hours, eastbound Hallandale Beach Blvd. was shut down completely as crews continued to work.

"The reason we shut down the area was to keep our units back, at a distance, and at the same time, keep citizens at a distance to make sure there was no secondary incident," Stover said.

The other two victims were taken to the hospital as well and were treated for minor injuries.

The cause of the explosion was not immediately known.