SAN DIEGO (AP) - U.S. Border Patrol agents stumbled on a cross-border tunnel after it caved in and the asphalt roadway above it collapsed, officials said.

The 35-foot-long tunnel, which was found Monday, ended in a patch of vacant land on the U.S. side near the San Ysidro port of entry, said Lauren Mack, a spokeswoman for U.S. Immigration and Customs Enforcement, the investigative arm of the Department of Homeland Security.

The primitive tunnel was about 3 feet by 3 feet and appeared to have been used recently, she said.

The exit on the U.S. side was sealed with sandbags and metal strips, Mack said. Authorities did not know when it was built and whether it was used for smuggling drugs or people, she said.

The tunnel is across the border from an area that is either owned or leased by Mexican Customs.

No arrests were made.

Also on Monday, authorities in Nogales, Ariz. discovered a 40-foot-long border tunnel. Two men were arrested as they hauled 300 pounds of marijuana from the tunnel, authorities said.

More than a dozen tunnels have been found along the southwestern border in recent years.

In 2003, Border Patrol agents found a tunnel that originated in a private house in Mexico and ended in a parking lot on the U.S. side. A year later, U.S. authorities found evidence that someone was trying to rebuild the passageway.

"We do believe it's a trend where smugglers are attempting to go underground after 9/11 as a result of the heightened security," Mack said.