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SAN DIEGO — More than 300 pounds of methamphetamine were discovered hidden inside objects made of stone Tuesday at a cargo facility in Otay Mesa, authorities said.

According to the U.S. Customs and Border Protection, the drugs were discovered after a 38-year-old man pulled a tractor with an attached trailer into the facility’s cargo port. The cargo, which appeared to consist of items made of stone, was taken for an in-depth inspection, where a canine alerted officers to potential suspicious materials inside.

CBP officers then broke one of the stone items open, revealing a wooden box inside. Officers said they found a substance that field-tested positive for meth inside the box.

Officers said they found 136 wrapped packages of meth weighing about 311 pounds hidden inside the cargo. The packages were worth an estimated $590,000.

The driver, a Mexican national, was arrested and taken to the Metropolitan Correctional Center.