Moments after the twin towers were struck, customs agents there stopped all 42,000 cars in all 24 lanes and inspected every single one of them.
"We began doing 100 percent searches of all the vehicles, checking compartments, trunks, hoods," says San Ysidro Port Director Oscar Priciado.
The agents found not a single ounce of dope. For three days, frustrated Mexican drug runners stopped every one of their shipments north while they watched border agents crack down with a vengeance.
"Immediately after the events of Sept. 11, we went to Alert Level One, which is the highest state of alert for the U.S. Customs Service," says Jason Ahern, director of field operations for U.S. Customs.
They remain at that level today.
And although the drug cartels have tried to resume business as usual, the added dogs, manpower, scrutiny and suspicion are paying off. Right after Sept. 11, heroin seizures at San Ysidro alone were up, 16-fold.
Customs agents are calling it the "Terrorist Dividend" and the lesson, they believe, is simple: As you crack down on the borders to look for terrorists, the bonus is that you'll find more drugs as well.
Level One Alert status means no more 15-second cursory inspections at the border. It means tougher searches areas other than the actual border, and it also means more random "blitz" searches where agents open up every vehicle.
The cartels are finding it even tougher through the air. Any suspicious aircraft crossing the border these days gets quick attention.
"We can launch the F-16 Interceptors," says Joseph Bendig, of Interdiction Control Center. "We'll go through the air defense sector ... ask them and they'll do the actual launch."
At sea, it's the same. The cartels tried at first to circumvent the tighter land borders by using fast boats on the high seas, but Coast Guard seizures are now up as well. And in one extraordinary case, drug dealers actually scuttled a freighter to avoid being caught with the evidence.
In San Ysidro, it's all part of the same game, only the name has changed.
"Our number one priority is terrorism," says Preciado. "Drugs are second to that."
And if you end up striking a blow against both, they reason, then so much the better.