U.S. and Mexico authorities are investigating the purchase by U.S. companies of millions of dollars worth of petroleum products stolen from Mexican national oil company Pemex.

On Tuesday, the U.S. Immigration and Customs Enforcement Agency will host a news conference with U.S. Department of Homeland Security and Mexican officials in San Antonio to discuss the scheme. The U.S. is returning $2.4 million in funds generated from smuggling to the Mexican government, a spokeswoman for ICE said.

A Houston executive pleaded guilty in connection with investigation after admitting that he and others conspired to buy the stolen products, which were loaded onto a barge in Brownsville, Texas, said Nancy Herrera, Executive Assistant U.S. Attorney in Houston. "We're working with the Mexican authorities," Ms. Herrera said.

Donald Schroeder, the president of Trammo Petroleum, is set to be sentenced in December. Mr. Schroeder, court documents say, purchased the product knowing it to be stolen. Mr. Schroeder's attorney could not be reached for comment.

Ms. Herrera declined to identify what U.S. companies bought the stolen products from Mr. Schroeder. Court filings describe the buyers as "larger companies."

The investigation is ongoing, though, Herrera declined to say if more indictments are pending.

In April, Mexican security forces arrested the group that allegedly made $46 million in the past two years smuggling stolen petroleum condensates to the U.S. The organization is linked to the Gulf drug cartel.

Last month, Mexico's Secretariat of Public Administration and other agencies began investigating Pemex employees to determine whether they conspired with the alleged smugglers.