By Gabriel Stargardter and David Alire Garcia

MEXICO CITY (Reuters) - Mexico's government vowed on Friday to find out whether an explosion that killed 33 people at the headquarters of its state-run oil monopoly Pemex was a deliberate attack or yet another stain on the company's poor safety record.

Rescue workers continued to pull bodies from the debris on Friday and officials said the search would continue until everyone inside the Mexico City building was accounted for.

Government officials have refused to speculate over what caused the explosion on Thursday but said they were keeping an open mind and had deployed large teams of experts to pore through the wreckage.

"The government is determined to find out the truth, whatever that may be ... whether it was an accident, negligence or an attack, whatever," Attorney General Jesus Murillo said on Friday evening. "We are not going to rule out anything."

He said the explosion did not cause a fire but refused to be drawn on what that implied about the cause.

The blast at Pemex's complex in the capital killed at least 33 people and a further 121 were injured. The scenes of chaos have dealt another blow to Pemex's image, just as Mexico's new government is seeking to open up the oil industry to more private investment.

Speculation over the cause has ranged from a bomb attack, to a gas leak, to a boiler blowing up.

"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," Pemex's chief executive, Emilio Lozoya, said on Friday.

Pemex, which was created when Mexico nationalized its oil industry in 1938, is a symbol of self-sufficiency but it has also been blighted by corruption, inefficiency and frequent accidents costing hundreds of lives.

The latest Pemex disaster is one of the first serious tests for President Enrique Pena Nieto, who took office in December saying overhauling the company was a top priority.

Investors have been closely following how far he will go in enticing private capital to boost flagging oil output in a country that is the world's No. 7 producer.

"This incident speaks very poorly of the image of Pemex management, and that's interpreted as additional risk in the market," said Miriam Grunstein, an energy researcher at Mexico's CIDE think tank.

A Pemex official said the damaged area of Pemex's headquarters was used for human resources in the corporate and refining divisions. It did not have a boiler or gas installations, the official said.

(Additional reporting by Adriana Barrera, Simon Gardner, Krista Hughes and Tim Gaynor in Phoenix; Writing by Dave Graham; Editing by Kieran Murray, Louise Ireland, Vicki Allen, Eric Beech and Lisa Shumaker)