LAREDO - The biggest commercial port on the frontier with Mexico is a favorite for drug traffickers and other contraband smugglers, and now federal agents here say they've uncovered a new scheme: fraudulent imports to the United States of fake silver coins used to claim $800 million in silver bullion sent back to Mexico.

The long-running scam cost Mexico more than $136 million in tariffs, according to court records.

Four people have been arrested, including Ricardo Ruvalcaba Plascencia, a native of Mexico who owns mansions in Laredo and San Antonio. The men are charged with conspiracy to commit wire fraud, smuggling goods from the U.S., falsely classifying goods, conspiring to defraud the U.S. and conspiring to launder money. The punishment for those charges ranges from a maximum of two years in prison for falsely classifying goods to 20 years in prison for money laundering.

U.S. Immigration and Customs Enforcement agents say the suspects avoided paying taxes on the silver bullion shipped from this country into Mexico by using dummy shipments of coins, shell companies and fraudulent paperwork. From 2008 to 2012, they accounted for 98 percent of all the silver shipped from the U.S. to Mexico.

The fraudulent importation of coins began in a warehouse that Ruvalcaba owns in a Laredo industrial park, federal investigators allege.

Avoided tariffs

A pair of U.S. companies filed paperwork with Mexican customs officials saying the coins contained silver and would be melted into bullion in the U.S. Later, when an El Paso company shipped bullion from the U.S. to Mexico, it claimed the silver came from those Mexican coins, so no tariffs were due, according to court documents.

In reality, the coins were dummies made of copper and nickel, according to the documents. Ruvalcaba and his co-conspirators wired millions of dollars from Mexico to buy silver bullion from U.S. suppliers, then avoided paying taxes on it by convincing Mexican authorities that the silver had come from the coins, investigators allege.

"(Homeland Security Investigations) has discovered that from May 15, 2008, to April 1, 2014, Ricardo Ruvalcaba and others have purchased over $800 million of silver bullion from three silver companies in the U.S.," agent Joseph Spielman wrote in an affidavit. "This silver bullion, when exported to Mexico, was also undervalued in Mexican customs documentation to avoid paying Mexican taxes and duties."

Mike McCrum, Ruvalcaba's lawyer, said his client's company had cleared the trade with customs.

Brother also charged

"We believe there has been a misunderstanding somewhere along the way," McCrum said. "We are hopeful that these charges will be dismissed, but in any event are confident our clients are not guilty."

Federal agents have also charged Juan Carlos Ruvalcaba Plascencia, Ricardo's brother; Victor Guadalupe Loera Garcia, who owns a precious-metals company that was also implicated in the alleged scheme; and Joaquin Hernandez Veliz, an employee of a Mexican company who investigators say helped package and ship silver bars from the U.S. to Mexico and coins from Mexico to the U.S.

Lawyers for the other defendants wouldn't comment.