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PHOENIX – A complicated tunnel has been found stretching almost a quarter of a mile from Mexico into the United States by Homeland Security Investigations.

Authorities said the tunnel was intended for smuggling and ran from a neighborhood in Mexico to a suburban neighborhood of San Luis, Arizona. The tunnel was discovered before there was access created on the U.S. side.

“This appears to be the most sophisticated tunnel in U.S. history, and certainly the most sophisticated I’ve seen in my career,” said Carl E. Landrum, acting chief patrol agent with the Border Patrol’s Yuma Sector.

The tunnel measured 3 feet wide and 4 feet high and was equipped with a fully developed ventilation system, water lines, electrical wiring, a rail system, extensive reinforcement and shoring, known to be commonly used in sophisticated subterranean tunnels.

Homeland Security Investigations found the tunnel in late July when a sinkhole appeared between the primary and secondary border fences. A camera was sent 25 feet (7.6 meters) underground after federal agents discovered a sinkhole in the area of a tunnel investigation, authorities said.

In January, a tunnel was discovered in the San Diego area that was more than 14 football fields in length that stretched into an industrial site in Tijuana, Mexico.

By federal law, U.S. authorities must fill the U.S. side of tunnels with concrete after they are discovered.

The Associated Press contributed to this story.