Federal authorities in San Diego have made one of the largest marijuana seizures in the United States, confiscating 20 tons of pot near an underground tunnel connecting warehouses on either side of California's border with Mexico, officials said Wednesday.

Mexican authorities seized another four tons of pot from the warehouse on their side of the border. Officials said the lightening-speed, 12-hour operation started Tuesday night when U.S. authorities watching a warehouse under surveillance followed a tractor-trailer as it left the building.

U.S. Immigration and Customs Enforcement agents called in the California Highway Patrol, whose officers stopped the rig near Temecula and authorities found 10 tons of marijuana inside. The driver, a U.S. citizen, and his Mexican wife were arrested and will be arraigned in San Diego on Thursday.

Authorities quickly obtained a federal search warrant to go inside the warehouse, where they discovered 10 to 15 tons of marijuana — worth an estimated $20 million, said ICE director John Morton.

They also found a wood door that opened to a hole leading to the tunnel, running the length of six football fields, under the border and into a warehouse in Mexico, Morton said. Officials found packets being ready to be moved into the United States from the tunnel, which had lighting, ventilation and a rail system to send loads of illegal drugs into California.

"This is not a mom-and-pop operation. This is obviously the work of a cartel," said Morton, who held a news conference outside the warehouse in an industrial park near the Otay Mesa truck crossing, across from Tijuana.

Officials said the seizure was the largest ever in California and was believed to be the second-largest in the U.S. Drug Enforcement Administration agents seized 33 tons of marijuana in Oregon in 2008, DEA special agent Ralph W. Partridge said.