Immigration and Customs Enforcement said yesterday that the passage was recently completed. Officials didn't elaborate in a brief statement.
Mexican news organisations published photos that show a deep, well-built tunnel near the Tijuana airport. Alberto Capella, Tijuana's public safety chief, told The Associated Press that a law enforcement operation was underway and he couldn't provide details.
Dozens of tunnels have been discovered along the border in recent years, designed largely to smuggle marijuana to the United States.