CARRIZO SPRINGS, Texas -- Workers at an oil rig just south of the city struck a pocket of gas, prompting an explosion Thursday morning about 10 a.m., firefighters said.

Five workers were injured, two of them critically, in the explosion. The two workers most injured were airlifted to Brooke Army Medical Center in San Antonio for further treatment.

A third worker is still being treated at the Dimmitt County Hospital, but may be transported to San Antonio later for further treatment, medical officials said.

A crew from the Premium Oil Company was drilling for oil at the No. 9 rig off of FM 186 when workers hit a pocket of gas as they were drilling, and it caught fire, according to fire investigators.

Work crews from Houston were expected to arrive sometime Thursday afternoon to help extinguish the flames. At its peak, witnesses reported seeing the smoke plume from 20 miles away.