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SAN DIEGO — The longest cross-border tunnel was recently discovered by authorities in San Diego, the United States Border Patrol said Wednesday.

The tunnel is approximately 4,309 feet long, with one off-shoot from the main tunnel that extends several hundred feet before abruptly ending. The tunnel is about 5.5 feet in height and 2 feet in width, authorities said.

Border Patrol agents said the tunnel featured an air ventilation system, a drainage system, a rail system and an elevator at its entrance when it was operational. It wasn’t in use at the time authorities discovered it, agents said.

The tunnel was initially uncovered in the Otay Mesa area in August. With help from Mexican authorities and intelligence gathering efforts, Border Patrol agents and Department of Homeland Security officials said they were able to determine over the following months how far the tunnel extended into Tijuana.

According to Lance Lenoir, a Border Patrol agent with the San Diego Sector’s tunnel team, the tunnel was most likely used to transport illegal materials into the United States with help from the rail system built inside.

“In my experience, this is exclusively a drug tunnel,” Lenoir said.

No arrests have been made in connection with the tunnel’s construction. Authorities said it was unclear how long the tunnel was operational and said the investigation into its construction and use was ongoing.

Border Patrol agents have shared news of a handful of tunnel discoveries in recent years. One tunnel unearthed in 2016 stretched nearly a half-mile between Otay Mesa and Tijuana and was outfitted with lights, a ventilation system and an elevator.

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