* Explosion kills at least 32, traps others, injures more than 100
* President courting private investment for state monopoly
* Pemex a byword in Mexico for accidents and oil theft (Adds details, background)
By Gabriel Stargardter
MEXICO CITY, Feb 1 (Reuters) - Rescue workers on Friday pulled out more bodies from debris at the headquarters of Mexican state oil giant Pemex, where an explosion that killed at least 32 people has thrown a spotlight back on the state-run company's poor safety record.
Scenes of confusion and chaos outside the downtown tower block in Mexico City have dealt another blow to Pemex's image just as Mexico's new president is seeking to court outside investment for the 75-year-old monopoly.
Thursday's blast ripped through a Pemex building next to the 50-story skyscraper, and chief executive officer Emilio Lozoya said the number of confirmed dead now stood at 32, up from 25 overnight. A further 121 were injured, he added.
It was not clear what caused the mid-afternoon explosion, Lozoya said.
"A fatal incident like yesterday's cannot be explained in two hours, we are working with the best teams in Mexico and from overseas, we will not speculate," he told a news conference.
Pemex, both a symbol of Mexican self-sufficiency and a byword for security glitches, oil theft and frequent accidents, has been hamstrung by inefficiency, union corruption and a series of safety failures costing hundreds of lives.
Investigators and rescue teams picked through shattered glass and concrete to look for survivors and discover what caused one of the worst explosions to hit Pemex in recent years.
Officials have been unable to say how many people may still be trapped within the wreckage of the office block. A military paramedic at the scene said there were likely many, and that he expected the death toll to continue increasing.
A government official, speaking on condition of anonymity, said a preliminary line of inquiry suggested a gas boiler had blown up in the building to the side of the main tower that houses administrative offices. However, he stressed nothing had been determined for sure.
Investigators have cordoned off the blast site, and a local Red Cross official, Isaac Oxenhaut, said the ceiling had collapsed in three lower stories of the building.
Lozoya said the four floors worst affected by the explosion normally had about 200 to 250 people working on them. That compared with about 10,000 staff in the entire Pemex complex.
The blast followed a September fire at a Pemex gas facility near the northern city of Reynosa which killed 30 people. More than 300 were killed when a Pemex natural gas plant on the outskirts of Mexico City blew up in 1984.
Eight years later, about 200 people were killed and 1,500 injured after a series of underground gas explosions in Guadalajara, Mexico's second biggest city. An official investigation found Pemex was partly to blame.
Shortly after the blast, President Enrique Pena Nieto was at the scene, vowing to discover how it happened.
"We will work exhaustively to investigate exactly what took place, and if there are people responsible, to apply the force of the law on them," he said.
Pemex said the blast would not affect operations, but concern in the government was evident as top military officials, the attorney general and the energy minister joined Interior Minister Miguel Angel Osorio Chong for a news conference.
"I have issued instructions to the relevant authorities to convene national and international experts to help in the investigations," Osorio Chong said late on Thursday. He later noted that the number of casualties could still climb.
Whatever caused it, the deaths and destruction will put the spotlight back on safety at Pemex, which only a couple of hours before the explosion had issued a statement on Twitter saying the company had managed to improve its record on accidents.
Pena Nieto has said he is giving top priority to reforming the company this year, though he has yet to reveal details of the plan, which already faces opposition from the left.
Both Pena Nieto and his finance minister were this week at pains to stress the company will not be privatized. (Writing by Dave Graham; Editing by Louise Ireland, Vicki Allen and Jackie Frank)