There's no lying to Eugenio Garza Jr., though people often try. Sometimes he sniffs out trouble before people even utter a word.

Garza, the U.S. Customs and Border Protection port director here, has spent more than a quarter-century on the boundary between the United States and Mexico, honing instincts that allow him to sort the day-trippers from the deceitful with little more than a glance. Not too long ago, he dropped in on a border inspection post to brief an officer, happened to look down at the driver of a waiting car and instantly ordered a search -- it turned up 100 pounds of marijuana in the gas tank and side panels.

"You have about 22 seconds to figure out whether someone is fully telling the truth," Garza said. "A good agent can just sense when something is not right."

That intuition takes years to develop, but today there is not time to do things the old-fashioned way. The threat of terrorism has added new urgency to the challenge of monitoring U.S. borders. The agency has already fortified the United States' ports of entry with an arsenal of sophisticated computer systems and is wooing hundreds of new recruits. Now, in a secure facility in Georgia, Customs and Border Protection is coaching its agents to use the ancient weapon Garza employs every day: gut instinct.

The three-day class, "Deception Detection and Eliciting Responses," draws on behavioral science to refine agents' interviewing technique and sharpen their ability to pick up on a lie. Agency officials are tight-lipped about the course, but outside experts describe a field of science focused on the unintended signals that people often send out when they are lying.

Changes in breathing, the rising pitch of a voice, the way someone moves -- or does not -- can all signal the stress generated by trying to hide something.

For example, people who are telling lies often unconsciously lean back, away from the person they are lying to. And, surprisingly often, a person falsely denying something will unwittingly nod his head up and down, in an unspoken acknowledgment of the truth.

Systematic research into verbal and nonverbal clues to deception began in the late 1960s, when scientists herded college students into labs to examine the physiological chain reactions that start when people fib. Other studies used film to minutely analyze the physical ways people move their faces and bodies.

Now, after almost four years of post-Sept. 11, 2001, emphasis on complex legal and technological strategies for preventing terrorist attacks, almost every major federal security agency is exploring new developments in deception detection, looking for ways to capitalize on skills that are second nature to seasoned police officers.

Customs and Border Protection started training its front-line officers in February. The classes, developed in-house, emphasize counterterrorism and cross-cultural communication. Eventually, 30,000 agency employees will be trained.

High over the muddy green Rio Grande, a blackened brass plaque on the Lincoln-Juarez Bridge marks the point where Mexico ends and the United States begins. Just over 100 feet into U.S. territory, the U.S. Customs and Border Protection gateway curves across the road.

Vehicles in the 12 lanes must pass through highly sensitive radiation-detection portals designed to intercept cargo that could be used to deadly ends. For commercial truck drivers who cross the border frequently and have been screened by the U.S. government, biometric devices match face to fingerprint to name in seconds. U.S. agents and their drug-sniffing dogs thread through the waiting cars.

And at the gate, Border Patrol officers and customs agents wait to conduct the personal interview. About 18,000 cars drive through this port of entry every day.

"We can't use the technology on everybody -- otherwise we would have lines down to Mexico," said Ramon Juarez, assistant port director.

"It's the human-nature element that's key," added the port's chief officer, Jesus Saldivar.

Ask Saldivar, Juarez and port director Garza to explain what tips them off, and they can't really do it. They "just saw something," or it was "just the way someone responded."

The process of introducing behavioral science into the training of law enforcement officers starts with the fact that researchers have identified four basic ways people signal discomfort. These signals do not necessarily indicate a lie in progress, but they do tell an observer that people may be hiding their true feelings.

First, the nervous system kicks in -- the heart rate quickens, hair rises, sweat glands start pumping, breathing speeds up, skin flushes pink or blanches pale.

The way people speak may change, with words tumbling out more quickly or the pitch of the voice climbing. They may clutter their speech with "ums" or verbally distance themselves from the subject being discussed -- saying, for instance, that "the glass was broken," not "I broke the glass."

Verbal content is another signal of discomfort. Stories that are too neat and linear raise a red flag. Truthful answers tend to jump backward and forward in time, and they often include an element of self-doubt.

And then there is the face, which experts liken to a leaky sieve of information. People blink more often when they are emotionally stressed. Micro-expressions, split-second flashes of concealed emotion, can be a more accurate guide, and one that works across cultures.

Agents interview border crossers at Nuevo Laredo, a point between Laredo, Tex., and Tamaulipas, Mexico.