The largest seizure ever of cocaine at the U.S.-Mexican border, a nearly 4-ton bust, was made after customs inspectors noted that a propane tanker truck was unusually overweight, authorities said Friday.

The 3.85-ton cocaine haul, worth about $262 million on the street, set a record for a border cocaine seizure, U.S. Customs spokeswoman Bobbie Cassidy.The 18-wheel Hidro Gas propane truck, headed into the United States from Juarez, Mexico, was stopped Wednesday at the Otay Mesa border station 10 miles east of San Diego, after the customs inspectors noticed it was 7,400 pounds overweight.

Propane trucks normally carry full loads down to Mexico and come back empty, Cassidy said.

After drug-sniffing dogs indicated the presence of narcotics, U.S. Customs and Drug Enforcement Administration agents spent the remainder of Wednesday and most of Thursday, 30 hours in all, bleeding propane from the tanker truck.

Once empty of propane, the cylindrical truck yielded about 7,700 pounds of cocaine, wrapped in roughly 2-pound plastic packets, Cassidy said.

"It was stacked to the gills," she said.

The driver remained at large Friday afternoon.