GUADALAJARA, Mexico (AP) _ The state-owned oil company is a prime suspect in explosions that killed at least 176 people in southwestern Mexico, an investigator says.

President Carlos Salinas de Gortari on Thursday afternoon gave the Federal District Attorney's Office 72 hours to determine whether ''criminal negligence of public servants'' was involved.

Investigators conducted interviews through the night and into the early hours this morning. Among those they questioned were officials of Pemex, the state fuel company. Pemex has several fuel storage tanks in the Reforma area of Guadalajara, which was leveled by Wednesday's blasts.

A chemical engineer who heads a state investigatory team, Esequiel Mendez, said late Thursday that Pemex is one of the main suspects.

Pemex has denied responsibility.

People left homeless or wounded by the loss of loved ones had no reservations about blaming the government.

''They cover up,'' said taxi driver Alfonson Gomez, who had lived in Reforma. ''The government does that all the time.''

Salinas' party has often been accused of corruption during its 63 years in power.

If the investigation determines that Pemex was to blame for Wednesday's blast, several high-level federal and state officials may be fired.

The bloodletting began late Thursday with the announcement that Gualberto Limon, director of the municipal sewer system, requested a leave of absence. Limon was one of the three state officials the governor had said he was holding responsible for the disaster.

About an hour later, Mayor Enrique Dau Flores told reporters he also was requesting to step down while the investigation was going on.

Pemex issued a statement hours after the blasts, blaming them on a leak of hexane gas, a volatile solvent, by a cooking-oil manufacturer.

But Thursday, Pemex suspended natural gas deliveries to industrial customers, closed its gas stations in Reforma and sent workers into the area to pump water out of sewer lines. Francisco Rojas, the company's director- general, described the moves as a ''safety precaution.''

State government officials, like Pemex at first quick to point to cooking oil companies, backed off Thursday and said they would wait for the results of an investigation.

Local newspapers quoted chemical engineers as saying it would have taken several tons of hexane gas to cause the explosions that ripped the sewer lines.