SAN DIEGO -- Border Patrol agents discovered a 65-foot-long tunnel about 3 miles west of the San Ysidro Port of Entry, it was reported today.

Agents looking for illegal aliens found the entrance to the tunnel on a rocky hillside last week, the San Diego Union-Tribune reported.

The tunnel, which stops just short of the U.S.-Mexico border, appeared to be under construction and may have been abandoned, Border Patrol Agent Ben Bauman said.

"The soil is fairly unstable in the area," Bauman said. "It may have been too difficult to continue construction."

Bauman said authorities believe the tunnel was being built for drug smuggling, although agents who set up surveillance around its entrance did not see any activity and no suspects were arrested.

The tunnel is on county government property, Bauman said, adding that it will be up to county officials to seal the entrance or fill it up.

Only last month, construction crews in rural East County closed down a cross-border tunnel that authorities said was used by associates of the Arellano Felix drug cartel to smuggle marijuana and cocaine into the United States.

DEA agents, acting on an anonymous tip, found the tunnel Feb. 27 while executing a search warrant at a house about 60 miles east of San Diego in the U.S. community of Tierra del Sol near Boulevard.

The 1,000-foot tunnel began in Mexico on the outskirts of Tecate under a fireplace in a ranch house located about 200 feet from the border fence. It ended in an unoccupied two-story house in Tierra del Sol.