Puzzled federal agents tried Tuesday to discover who dug an 85-foot-long tunnel that started in Mexico, ran directly underneath a U.S. Customs Service parking lot in Southern Arizona, and was believed used at least once to smuggle drugs into the United States.

The tunnel, complete with wood bracing and electricity illegally tapped from a Customs employee parking lot, was found Sunday by a sharp-eyed government security guard patrolling the area, said Roger Maier, a Customs Service spokesman.