The U.S. and Mexican governments are probing a scheme to steal millions of dollars worth of crude oil and refined products from Mexico's state-owned oil company and sell it to U.S. refiners, a U.S. official said on Monday.

There is a cooperative effort by the United States and Mexican governments to investigate the theft of petroleum products from Mexico, said Nancy Herrera, spokeswoman for the U.S. Attorney's office in Houston.

The Immigration and Customs Enforcement agency, or ICE, will hold a news conference in San Antonio on Tuesday to announce that the United States will return $2.4 million in funds generated from oil smuggling to the Mexican government, the agency said.

The announcement comes on the heels of U.S. President Barack Obama's meetings over the weekend with Mexican President Felipe Calderon and Canadian Prime Minister Stephen Harper in Guadalajara, Mexico.

At least one U.S. energy industry executive has pleaded guilty in a scheme to steal about $2 million worth of petroleum products from Mexico's state oil monopoly Pemex and sell it to U.S. refiners.

Donald Schroeder, president of Houston-based Trammo Petroleum, pleaded guilty in May to smuggling stolen petroleum products -- including crude oil condensate -- from Pemex, according to court documents. Schroeder is scheduled to be sentenced in December.

Trammo Petroleum purchased the stolen petroleum products, which were shipped into the United States on barges and trucks, and then sold them to unspecified unnamed companies, according to documents filed by the U.S. government in May 2009.

He has admitted he and others were involved in shipping a barge of condensate worth $2 million from Brownsville (Texas) to a company near Houston, Herrera said.

Fuel theft is rampant in Mexico and costs state oil monopoly Pemex

more than $2 billion a year. Federal police last month raided the Pemex headquarters as part of an investigation into the thefts.

President Felipe Calderon last week accused the country's powerful drug gangs of being involved in the fuel thefts.

Mexican fuel thieves take advantage of Pemex's aging pipeline network to tap into pipes and siphon off fuel. The company located 396 illegal connections to its fuel pipelines in 2008, Pemex said.

Pemex officials admit they have no clear idea how much fuel is stolen every year.

(Additional reporting by Robert Campbell in Mexico City; Editing by Christian Wiessner)