AILSA CHANG, HOST:
And now to California, where authorities have announced the discovery of a tunnel running under the U.S.-Mexico border. It's more than 4,000 feet long, and it stretches from Tijuana to San Diego. Border Patrol says the tunnel, the longest ever discovered, was used to smuggle drugs and possibly people. Matt Hoffman from member station KPBS has more.
MATT HOFFMAN, BYLINE: Video provided by Customs and Border Protection shows a cramped tunnel, just 2 feet wide and 5 feet high, dug 70 feet below ground. It runs beneath a freeway and a series of border fences. John Callery of the Drug Enforcement Administration says it's a reminder of how serious the drug trafficking problem is and how many resources the U.S. puts into stopping it.
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JOHN CALLERY: As efforts to strengthen security on our southern border increase, Mexican cartels are forced underground to smuggle deadly drugs and other contraband into the United States.
HOFFMAN: The tunnel has electricity, ventilation and even a drainage system. On the floor are rails. Authorities haven't made any arrests or seizures since the tunnel was discovered last August, but they're confident it was used to move drugs.
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CALLERY: I have no doubt that if drugs were smuggled in this tunnel that it was heroin laced with fentanyl. I would - that's a 99-percentile answer on that.
HOFFMAN: Video from inside the tunnel shows murky brown water covering parts of the floor. It's mixed with floating debris, such as bottles and rope. Aaron Heitke with the San Diego Border Patrol says the tunnel was abandoned when they found it.
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AARON HEITKE: But evidence strongly suggests the tunnel was previously operational.
HOFFMAN: Border Patrol is not ruling out that the tunnel was also used for human smuggling. On the Mexico side, it started in Tijuana, near an industrial area. On the U.S. side, it ends near a warehouse district in an area called Otay Mesa, the opening there blocked with sandbags. It's unclear when the tunnel was built, but, Heitke says, it's length and complexity show just how much time and resources cartels are willing to spend to smuggle drugs.
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HEITKE: They're very well-funded, and they'll try any means they can to get their illicit cargo into the United States.
HOFFMAN: Heitke says the find was the outcome of a multiyear investigation in collaboration with Mexican authorities.
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HEITKE: Although no arrests or seizures have been made, the investigation is ongoing. And I am confident that the task force will aggressively pursue the criminals connected to this tunnel.
HOFFMAN: Before this discovery, the longest underground tunnel beneath the U.S.-Mexico border was also found in San Diego in 2014. That tunnel was nearly 3,000 feet long.
For NPR News, I am Matt Hoffman in Otay Mesa, Calif.
(SOUNDBITE OF MUSIC) Transcript provided by NPR, Copyright NPR.