The explosion was reported in the 13500 block of Orden Drive at about 6:30 a.m., a Downey Fire Department spokesman said.
It involved a tanker truck that was carrying about 4,000 gallons of tar used in roofing jobs, as well as smaller tanks of propane.
Both victims, contractors with AAA Roofing, were standing on a platform and heating asphalt to 500 degrees when the explosion occurred, according to Division Chief Brent Hayward with the Santa Fe Springs Fire Department.
Both victims burned in Santa Fe Springs were standing on platform when propane explosion happened. Terrible scene. pic.twitter.com/hg24VSMfIA— Marc Cota-Robles (@abc7marccr) May 2, 2016
The two roofers were found on the ground about 15 feet away from the platform.
One of the two victims was in critical condition after he suffered third-degree burns on 40 to 60 percent of his body. Hot asphalt also covered his face.
"He was conscious, he wasn't talking to us at the time," Hayward said.
The second victim suffered second-degree burns to his chest and abdomen, and suffered a shoulder injury from the fall. Both roofers were transported to UC Irvine Medical Center in critical condition.
The cause of the explosion remains under investigation.
"We're still trying to determine what kind of an explosion it was at this time, if it was a propane explosion or an asphalt vapor explosion," Hayward said.
The investigation is ongoing. Meanwhile, the victims' co-workers are reeling in the aftermath.
"They're obviously devastated. These guys work together a lot, much like the fire service. They all know each other, they all know their families, and it's an emotional event to see something like this happen. We don't see this every day," Hayward said.