Juan Jose Suarez, director of the state-owned Petroleos Mexicanos company, told local media earlier in the day that at least five workers had not been seen since the blast. On Tuesday, the company, known as Pemex, said in its Twitter account that a total of seven people were unaccounted for.
President Felipe Calderon said the quick reaction of emergency teams prevented a "real catastrophe," by controlling the fire before it reached the huge tanks of a neighboring gas processing plant.
The enormous fire Tuesday hit a distribution center near the border with Texas that handles natural gas coming in from wells and sends it to a processing plant next door.
"The timely response by oil workers, firefighters and the Mexican army was able to control the fire relatively quickly and avoid a real catastrophe of bigger proportions and greater damages if the fire had spread to the center for gas processing, which is right there," Calderon said in a speech in Mexico City.
The blast and ensuing fire left charred tanks and a mound of tangled steel at the walled plant near the border city of Reynosa, across from McAllen, Texas.
Two of the injured were reported in serious condition.
Dr. Jaime Urbina Rivera, deputy medical director of Hospital Materno Infantil de Reynosa just a few miles from the plant, said his hospital had received nine injured workers with first- and second-degree burns covering 10 percent to 40 percent of their bodies, with the burns concentrated on their backs and legs. They all arrived conscious, he said.
Pemex officials said the blast appeared to have been caused by an accidental leak, and there was no sign so far of sabotage.
The facility's perimeter walls, topped with razor wire as a security measure in a country that has seen thieves, saboteurs and drug gangs target oil installations, presented an obstacle for plant workers trying to flee.
Esteban Vazquez Huerta, 18, who was inside the plant when the fire erupted, managed to find a gap in the wire, scale a wall and escape. "We had to climb the wall from that side because the fire, the heat was reaching us," Vazquez Huerta said Wednesday as he stood outside the plant, waiting for word of missing co-workers.
Until the final moments before the explosion there was no sign anything was amiss, Vazquez Huerta said.
Pemex said workers from contracting firms, such as Vazquez Huerta, and its own employees were performing routine maintenance at the plant, where pipelines from gas wells in the Burgos basin converge. The plant feeds gas next door to separate liquid hydrocarbons from the gas. The production is for domestic Mexican use.
Vazquez Huerta said that suddenly the pipes where he was working, about 300 to 400 yards (meters) from the explosion, began to sound like they were repressurizing, after being closed for maintenance.
There was a blast and he and two co-workers began running. A second explosion knocked them to the ground, but they got up and continued running. They found a space along the back wall that wasn't topped with razor wire and boosted each other over.
Calderon said that the government will carry out an exhaustive investigation of the cause of the fire and that federal prosecutors will open a probe.
The blast forced the closure of the wells and the evacuation of people at ranches and homes within three miles (five kilometers) of the gas facility, which is about 12 miles (19 kilometers) southwest of Reynosa.
Calderon sent condolences to the victims' relatives and vowed to make sure those injured receive help.
Pipelines carrying gasoline and diesel in Mexico are frequently tapped by thieves looking to steal fuel, and those sometimes cause spills or explosions. But thieves seldom target gas pipelines.
In December 2010, authorities blamed oil thieves for an oil pipeline explosion in a central Mexico city near the capital that killed 28 people, including 13 children. The blast burned people and scorched homes, affecting 5,000 residents in an area six miles (10 kilometers) wide in San Martin Texmelucan.