MEXICO CITY — U.S. refineries bought millions of dollars worth of oil siphoned from Mexican government pipelines and smuggled across the border — in some cases by drug cartels expanding their reach.

At least one U.S. oil executive has pleaded guilty to a conspiracy that involved what prosecutors said was about $2 million in stolen Mexican oil, U.S. Justice Department officials confirmed to The Associated Press.

On Tuesday, the U.S. Homeland Security department is scheduled to hand over $2.4 million to Mexico's tax administration, the first batch of money seized during a joint investigation into smuggled oil that authorities expect to lead to more arrests and seizures.

"The United States is working with the Mexican government on the theft of oil," said Nancy Herrera, spokeswoman for the U.S. Attorney's office in Houston. "It's an ongoing investigation, with one indictment so far."

In that case, Donald Schroeder, president of Houston-based Trammo Petroleum, is scheduled to be sentenced in December after pleading guilty in May. Trammo's small firm profited about $150,000 in the scheme, in which various U.S. companies hauled stolen Mexican petroleum products across the border in trucks and barges, according to court records.

Schroeder's attorneys said in an e-mail that neither they nor their client would respond to AP's requests for comment.

In Mexico, federal police commissioner Rodrigo Esparza said the Zetas, a fierce drug gang aligned with the Gulf cartel, used false import documents to smuggle tanker loads of oil to unnamed U.S. refineries.

Earlier this year, the Mexican government froze 149 bank accounts after tracking more than $46 million in transfers related to the Zeta theft. The losses continue at a record rate, according to Mexico's state oil monopoly Petroleos Mexicanos, or Pemex.

In a surprising public acknowledgment, Mexican President Felipe Calderon said last week that drug cartels have extended their operations into the theft of oil, Mexico's leading source of foreign income which finances about 40 percent of the national budget.

"These are Mexican resources, and we do not have to sit back or turn a blind eye," Calderon said. "This is our national heritage and we must defend it."