CALEXICO, Calif. – Federal agents Friday discovered a clandestine tunnel that may have been built to smuggle drugs and possibly people from Mexico to the United States.

The tunnel was found on the U.S. side of the border fence in Calexico, about 120 miles east of San Diego, said Lauren Mack, a spokeswoman for the U.S. Department of Homeland Security. It was about 20 feet underground, measuring 3 feet in diameter and reinforced with wood and plastic. Agents found rubber tubing possibly intended for ventilation.

U.S. and Mexican authorities had not determined where the tunnel originated and ended – or even if it was completed, Mack said.

Federal authorities have discovered at least 12 tunnels along the U.S.-Mexico border since Sept. 11, 2001, some of them only partially completed, Mack said.

The latest discovery was made using surveying tools that Mack declined to describe in detail.

Last year, Homeland Security began using technology developed by geophysicists to pinpoint suspicious terrain. Previously, it relied on human intelligence and sheer luck, such as when a Border Patrol agent in Calexico struck a sink hole in 2003.