(Reuters) – Border Patrol agents discovered a tunnel running under the U.S.-Mexico border Saturday in San Diego, California apparently used for illegal immigration, authorities said.

Border Patrol Agent Eduardo Olmos said Saturday that agents detained about 30 people including Mexican and Chinese nationals. No drugs were found, he said.

When agents approached a group of people at the site early Saturday morning, some started running and others went back into the tunnel, leading to its discovery, Olmos said.

The tunnel began in a building in Tijuana, more than 300 feet (90 meters) south of the U.S.- Mexico border and ran past a fence at the Otay Mesa Port of Entry, KNSD-TV reported. Olmos could not confirm where the tunnel started or its length.

Subterranean tunnels found in the area are most commonly used for drug smuggling, according to the Customs and Border Patrol.

In 2016, federal agents said they seized a ton of cocaine and seven tons of marijuana smuggled through a tunnel in the same area that stretched a half mile beneath the border, which then was the longest one yet unearthed in California, Reuters previously reported.

Authorities said at the time that was the 13th underground passageway discovered along California’s border with Mexico since 2006.

Tunnels designed for drug smuggling are often more sophisticated and can include lighting, ventilation or rails that run as deep as 90 feet below the earth’s surface, Olmos said. Human smugglers tend to use more rudimentary tunnels, he said.

“This seems to be somewhere in between,” he said.

The latest tunnel find is under investigation by the San Diego Tunnel Task Force, led by U.S. Immigration and Customs Enforcement.