Murillo said experts have "not found any indication of fire."
That disaster was a major setback to a safety record that had been improving following a series of incidents in the 1980s and 1990s, according to company figures. The number of accidents per million hours worked dropped by more than half, from 1.06 in 2005 to 0.42 in 2010. That is in line with the international average of about 0.43 per million, according to the U.K.-based International Association of Oil and Gas Producers, which does not independently verify company numbers.
But Pemex acknowledged in a report that starting in late 2011, a series of smaller blasts and fires, mainly at refineries and petrochemical plants, had "seriously impacted" its safety rate. It said the rate of injuries per million hours had risen to 0.54.
As part of the federal government, Pemex is entirely responsible for inspecting its own buildings. A Pemex spokesman said the company could not immediately provide information about the affected building's safety record because of the workload generated by the response to the explosion.
Mexico City-based oil analyst David Shields said he was pessimistic that any one particular accident would have a real impact on the broader push for reform.
"What they really do is spark a certain amount of indignation, most of the time they don't result in changes," he said.
Before the pipeline blast in Reynosa, Pemex's last big accident was in 2007, when a sudden storm hit an offshore oil rig, killing 22 workers.
Thursday's explosion occurred at about 3:45 p.m., just as the administrative shift was about to end. It hit the basement and three floors, where as many as 250 people work, Lozoya said. The floors collapsed in the 14-story administrative building at the headquarters office complex, which was built starting in the late 1970s. Some 10,000 people work there daily. Lozoya said about 1,700 work in the building affected.
Gabriela Espinoza, 50, a Pemex secretary for 29 years, was on the second floor of the tower when she said she heard two loud explosions and a third smaller one.
"There was a very loud roar. It was very ugly," she said.
Lozoya said Pemex operations continue uninterrupted despite the blast and that the company is producing 2.57 million barrels of oil per day.
Pemex, created as a state-owned company in 1938, has nearly 150,000 employees, according to its website, with $111 billion in sales. Pena Nieto, who took office in December, has made Pemex reform the center of his platform, with a plan to pump new investment into a company whose profits feed much of Mexico's federal budget, but which has fallen behind other oil companies in production, technology and exploration.
AP writers Katherine Corcoran, Olga R. Rodriguez and Adriana Gomez contributed to this report.