The Defense Department says the tunnel two yards (meters) under the surface may have been used to cross the border into the United States. However, the department's statement Sunday does not say whether the tunnel actually reaches into U.S. territory or whether any entrance on the U.S. side has been found.
Such tunnels found previously in Nogales and elsewhere on the border have been used to move drugs or migrants.
The tunnel was discovered by soldiers Friday. The army says one man carrying an illegal weapon was detained at the scene.