San Diego ? A Mexican citizen was arrested on drug charges in the investigation into the longest tunnel ever found underneath the U.S.-Mexico border, U.S. officials said Monday.

The suspect was taken into custody Saturday by U.S. Immigration and Customs Enforcement agents. He appeared in federal court Monday on charges of conspiracy to import more than a ton of marijuana, which carries a mandatory minimum of 10 years in prison.

The 2,400-foot tunnel runs from a warehouse near the airport in Tijuana to a warehouse in San Diego. It was unclear how long it had been in operation, but more than two tons of marijuana were found inside last week.

A U.S. Immigration and Customs Enforcement agent looks down a shaft of tunnel that leads into Mexico from the U.S. side of a cross-border tunnel Monday in San Diego. U.S. and Mexican officials believe that the sophisticated tunnel that passes under the U.S.-Mexico border was used to smuggle drugs into the United States.

Authorities said the passage was 5 feet high and ran as deep as 90 feet below the surface. It had a concrete floor, lights along one of the hard soil walls, a groundwater pump, and pipes that circulated fresh air.

The man under arrest, Carlos Cardenas Calvillo, worked at the U.S. warehouse, which had been leased for the stated purpose of distributing produce, authorities said.

Cardenas told investigators that he began working in sales at the warehouse in October. Within a month, he said, he knew of the tunnel and knew that the warehouse was a front for drug deals, immigration authorities said.

The Drug Enforcement Administration has said it suspects Tijuana’s ruthless Arellano-Felix drug syndicate or another well-known cartel was involved.