Mexican authorities have arrested four executives of a Mexico City firm involved in a U.S. grand jury investigation of charges that American companies made massive payoffs to officials of Mexico's government-owned oil company.

According to a statement issued yesterday by the Mexican attorney general's office, the four were charged on Sunday with evading taxes on at least $45 million that was sent by international money orders to three Swiss banks and one U.S. bank. Sources in the attorney general's office said they believe some of that money was paid to officials of Petroleos Mexicanos (Pemex).

Arrested were four of five members of the board of directors of Grupo Industrial Delta, a brokerage company that represented U.S. firms in dealings with Pemex. Police were unable to locate the company's president, Ricardo Garcia Beltran, who sources say is a target of the U.S. grand jury investigation of the bribery charges. Sources close to the investigation in Mexico said they believe Garcia Beltran is hiding in Houston.

The U.S. investigation covers the period in the late 1970s when oil prices were rising and Mexico was buying huge amounts of equipment to increase oil and gas production. American firms allegedly bribed Pemex officials in an effort to win lucrative contracts awarded by the Mexicans, sources say.

Sources at the Mexican attorney general's office said the investigation continues on what happened to the money allegedly sent to Switzerland by Grupo Delta. The money was transferred via money orders bought in Multibanco Comermex, a Mexican bank. "We have the numbers of the international money orders, and we have proof of deposit," the sources said.

In its statement, the attorney general's office--which has worked closely with the U.S. Justice Department--said that Delta signed a brokerage contract in 1978 with a U.S. company, Crawford Disc Inc., to obtain sales contracts for equipment destined for Pemex.

"Delta received as pay for these contracts 7 percent of their value and wrote invoices without entering the income in their bookkeeping. The money was deposited in bank accounts other than those of the company itself." It was a case of "double invoicing," the statement said.

Crawford Disc Inc. is part of Crawford Enterprises Inc., a relatively small Houston firm that is one target of the grand jury investigation, which has spanned several years and is being handled by the Department of Justice and by U.S. Customs. Crawford said in a statement released last week that it has denied allegations of wrongdoing and that Pemex and the Mexican government had looked into similar charges and found no wrongdoing in the award of Pemex contracts to Crawford.

Grupo Delta has sued Crawford in U.S. District Court in Houston, charging that Crawford still owes the firm $25 million in agent fees. Crawford has denied the charge.

Grupo Delta, which is registered as a broker with the Mexican finance ministry, is believed to have closed deals worth at least $647 million, the statement said. But the magnitude of the business it arranged between U.S. companies and Pemex, and therefore the size of its alleged tax evasion, is believed to be much higher.