Huge Cocaine Seizure at Border Crossing a Record : Crime: A two-nation manhunt for the driver of the tanker truck, who had been released prior to the discovery of the drugs, is conducted.

Federal authorities, having completed the unloading of almost 4 tons of cocaine discovered this week in a propane tanker truck en route from Mexico, on Friday declared it the largest cocaine seizure ever along a U.S. land border crossing.

The 7,702-pound haul--with an estimated street value of $262.5 million--is believed to be the second-largest ever seized in California. A mammoth cache of 21 tons (42,000 pounds) was discovered last year in the San Fernando Valley community of Sylmar.

The Sylmar load had been smuggled into the United States from the El Paso, Tex., border area, a fact that experts say underscores the border’s role as a conduit for illicit substances headed for the U.S. market.

U.S. and Mexican officials were conducting a two-nation manhunt Friday for the truck driver, who was allowed to leave the Otay Mesa inspection station area when customs officials were unable to readily determine whether drugs were inside the suspicious truck.

The driver, a Mexican whose name was not released, was detained for at least eight hours before he was allowed to go free Wednesday afternoon, Customs officials said. Despite the ongoing search of his vehicle, authorities said that there was no legal basis to detain the man any longer.

“We can’t hold someone indefinitely at the border,” said Thomas W. Hardy, acting district director for the U.S. Customs Service in San Diego. “I can’t say there was an error; we’re looking into that.”

The search to discover the drugs dragged on for almost 1 1/2 days, authorities said. Inspectors first took apart the truck’s 18 tires and later bled excess propane, which is highly volatile, from the tank. Investigators did not discover the cocaine until 2:30 p.m. Thursday--about 24 hours after the driver had been permitted to leave and some 32 hours after the vehicle had initially pulled into the border entry. The drugs were finally all unloaded at 4 a.m. Friday.

Inspectors first became suspicious, officials said, when narcotics-sniffing dogs stationed at the facility paid attention to the vehicle’s tires, indicating that there might be drugs concealed in the wheels. After weighing, it was determined that the truck, which was supposedly empty, weighed more than it should. Authorities said there was no advance information that the vehicle might be ferrying contraband.

The drugs were wrapped in 3,501 packages, or bricks, each weighing about 1 kilogram, or 2.2 pounds. Among the wrappings used, authorities said, was a newspaper from Medellin, Colombia, base of the legendary drug cartel.

U.S. officials would not comment when asked about the likely destination of the cocaine. The substance was probably loaded onto the truck in Mexico, within five miles of the U.S.-Mexico border, said Ron D’Ulisse, spokesman in San Diego for the U.S. Drug Enforcement Administration.

The bust, officials said, illustrates the wide use that international traffickers are making of the U.S.-Mexico land border, which stretches for almost 2,000 miles from Texas to California. Mexico has long been a major transshipment point for South American cocaine, as well as a primary source of heroin, marijuana and other illicit substances favored by U.S. users.

Smugglers utilize both the vast unguarded spaces between border entries and attempt to sneak drugs in vehicles through the ports, hoping that understaffed inspection teams will not search their cargoes. Tens of thousands of cars and trucks enter the United States each day from Mexico, a fact that makes its impossible to search each vehicle. Tanker trucks frequently have been used to ferry marijuana and other illicit substances.

“They (the traffickers) never stop being innovative,” said Julius Beretta, special agent in charge of the U.S. Drug Enforcement Administration in San Diego. “Something like this leads you to wonder how many times it has happened (before). We just don’t know.”

Last year, 589,000 trucks--more than 1,613 per day--entered the United States at the Otay Mesa crossing, which is about 8 miles east of the larger, non-commercial port at San Ysidro.

The white tanker truck ferrying the cocaine normally carries propane gas for Hidro Gas Juarez, a firm in Ciudad Juarez, Mexico. Executives of the firm could not be contacted Friday by The Times. U.S. authorities said that the firm was cooperating in the investigation. U.S.-produced propane is regularly shipped in such tanker trucks to Mexico, where it is widely used for cooking and heating.

The company has had related problems before, a customs official said. In 1985, said Bobbie Cassidy, a customs spokeswoman in San Diego, inspectors discovered two separate loads of marijuana--80 pounds and 55 pounds--in the tires of two Hidro Gas Juarez trucks. She could not say if any prosecutions resulted from those seizures.

The most cocaine seized at a U.S. land border before this is believed to have been 3,589 pounds near El Paso in September, 1987, said John Miller, a customs spokesman in Los Angeles. The largest San Diego seizure before this was 1,920 pounds in 1988 from a truck at the Otay Mesa port, authorities said.