SAN DIEGO (Border Report) — Six people, all U.S. residents, have been arrested in connection with the discovery of a smuggling tunnel Friday in San Diego and Tijuana.

The cross-border tunnel, which Mexican authorities found first, begins in Tijuana and ends inside a warehouse just east of the Otay Mesa Port of Entry.

The tunnel measures 1,744 feet in length and is 61 feet deep, agents with Homeland Security Investigations said Monday. It measures 4 feet in diameter and has reinforced walls and ventilation, electric and rail systems.

Inside, investigators found 1,762 pounds of cocaine,164 pounds of meth and 3.5 pounds of heroin.

It was discovered on Friday by agents who were conducting surveillance on a “known drug stash house” in National City, just south of Downtown San Diego.

They followed two suspects who left the house in a vehicle and drove to the warehouse.

“Upon searching the warehouse, agents uncovered a sophisticated border tunnel exit point carved out on cement floor,” said U.S. Attorney Randy Grossman. “We allege our defendants were driving into garage and loading and dropping off cardboard boxes full of drugs to further the movement and distribution of drugs into the United States.”

The tunnel is the 91st in the Tijuana-San Diego area and the 272nd tunnel found along the entire border since 1993.

The six suspects were arraigned Monday afternoon in a federal courtroom, all six pleaded not guilty.