U.S. Border Patrol agents discovered on Saturday morning a tunnel near the Mexican border near San Diego that was used to smuggle people into the country, U.S. Customs and Border Protection said.

The discovery was made after CBP agents arrested 30 people who were in the U.S. illegally near the Otay Mesa port of entry.

"At around 1:12 a.m., Border Patrol agents encountered several people who had apparently just been smuggled into the U.S.," CBP said in a statement. "Agents searched the area and discovered a crude opening in the ground with a ladder inside and determined that is was a smuggling tunnel. The tunnel's exit is located just north of the secondary fence in the vicinity of the Otay Mesa port of entry."

The 30 individuals are currently in Border Patrol custody and are being questioned.

There were a total of 23 Chinese nationals (21 men, 2 women) and 7 Mexican nationals (4 men and 3 women), CBP said.

CBP said members of the San Diego Tunnel Task Force are coordinating closely with their law enforcement counterparts in Mexico on the investigation.

"Preliminarily it appears this latest tunnel may be an extension of an incomplete tunnel previously discovered and seized by Mexican authorities," CBP said.

The use of such tunnels is not new, but they have typically been used for drug smuggling.

"While subterranean tunnels are not a new occurrence along the California-Mexico border, they are more commonly utilized by transnational criminal organizations to smuggle narcotics," the CBP said. "However, as this case demonstrates, law enforcement has also identified instances where such tunnels were used to facilitate human smuggling."