Officials evacuated the complex and operations will cease there until further notice, the company said.
The state oil company's director, who was on a business trip in South Korea this week, said in a Twitter post that he would return to Mexico immediately.
"We will get to the bottom of the causes in close coordination with the authorities," Pemex Director General Emilio Lozoya Austin said. "At this time, attending to the injured is the priority."
Federal troops and rescue dogs were aiding in search efforts late Thursday night.
As they combed through the rubble, a Pemex executive's cell phone rang, Rangel said.
A man on the other end of the line said he was still trapped, and started to describe his location to rescuers.
"Unfortunately, we lost communication with him," Rangel said.
When they called back, his phone didn't ring.