U.S. officials handed to Mexican authorities Tuesday $2.4 million paid as restitution by an oil importer after the company president admitted knowingly buying and reselling petroleum products stolen from Mexico's oil monopoly.

Assistant Homeland Security Secretary John Morton attended an event in San Antonio with Mexico's tax administrator, Alfredo Gutierrez Mena, to mark the return of the cash paid by Houston-based Trammo Petroleum. Both men said the investigation was an example of the countries working together on cross-border crime.

"We are serious about going after organized crime, and we're serious about doing it in a coordinated and cooperative way," said Morton. "It's a strong signal that we mean business."

In addition to the restitution, Trammo agreed to pay a separate $2 million fine to the U.S. government, said Assistant U.S. Attorney Jim McAlister, the prosecutor on the case against Trammo Petroleum President Donald Schroeder.

Schroeder pleaded guilty in May to buying and reselling $2 million worth of condensate, a crude petroleum product used for blending, McAlister said.

According to court documents, Schroeder in January participated in a call with an unnamed oil company employee about the problem of importing stolen condensate. Nearly three weeks later, Schroeder arranged for a barge to be loaded with the stolen product in Brownsville and shipped for resale, the federal indictment says.

Schroeder's attorneys have declined to comment on the case.

Morton said that in June 2007, Immigration and Customs Enforcement agents were told about allegedly stolen petroleum products entering the U.S. from Mexico, and an investigation began. Only Mexico's state oil monopoly Petroleos Mexicanos, or Pemex, can sell and distribute petroleum products from Mexico, which is heavily dependent on oil sales to fund its budget.

Ten federal search warrants have been served on bank accounts in Texas as part of the investigation, though authorities declined to say whether more indictments were pending and were reluctant to talk about details of the case.

Companies like Trammo work as resellers of crude oil that goes to refiners. McAlister said it can be difficult for investigators to determine who in the process is aware of a stolen product's origins.

"Some are aware it's stolen, and some are not," he said.

Trammo Petroleum, a subsidiary of New York-based Transammonia AG, is a tiny player in the oil business. According to the company's Web site, it moved about 2.1 million barrels of crude oil in 2008, less than San Antonio-based Valero Energy Corp. can refine in a single day.

Officials at Tuesday's event declined to comment on how widespread oil theft is in Mexico, but Mexican President Felipe Calderon said last week that drug cartels have extended their operations to include stolen oil. Pemex officials are adding security measures and extra guards to catch the thieves but say the problem is growing.