The gem gang struck again last week, this time ramming and surrounding the car of two salesmen driving from the airport, then snatching bags of expensive jewelry from them at gunpoint. The latest heist, like so many others, was brazen, clever and fast. And, as usual, the masked bandits got away.

When he heard the news, detective Mike Woodings wearily added the ambush to the long list of similar cases that the Los Angeles Police Department is struggling to crack this year.

"No matter what we do," he said, "there's just a constant tide of these guys out there that we have to contend with."

Jewel thieves are running amok in Southern California, swiping fortunes in merchandise, baffling police and terrifying an industry to the point that some dealers and wholesalers are closing or moving.

But there may be nowhere to run: Across the West, and in cities as far-flung as Chicago, Houston and Miami, police also are reporting an increase in jewelry robberies that are being staged in virtually the same manner as those occurring here, by similar suspects. Instead of targeting stores, which are usually fortified with alarms and security cameras, groups of bandits are preying on jewelry salesmen where they are most vulnerable, the road.

There have been nearly three dozen such attacks in Los Angeles County this year, an unprecedented spree, with losses exceeding $13 million. In the past few weeks, several roadside gem heists also have been reported near San Diego and Phoenix. Investigators say the gangs study the routines of salesmen and then strike with military precision.

In one instance, a jewelry salesman apparently was followed here from Atlanta and then robbed. In another, a salesman who had a guard dog in his van returned from a brief highway stop for food to find his windows smashed, his dog put to sleep and his jewels gone. Even wholesalers traveling with bodyguards have been ambushed by men with automatic weapons.

"Dealers are worried sick now every time they get into their cars," said Frank Barcott, a retired police officer in Orange County whose security firm provides protection, especially for jewelers. "We know we're not dealing with kids. These guys are pros. They are targeting people they watch very, very closely and then getting them when they least suspect it."

Police in Los Angeles and elsewhere say their investigations of the heists strongly suggest that most, if not all, are the work of a well-organized crime ring run by Colombian nationals and based in Southern California. They also suspect that the shadowy group is recruiting and training scores of young men and women here and across the country to carry out the elaborately planned robberies, after which they and the stolen gems are quickly shuttled to another state or overseas. Police have played a cat-and-mouse game with the thieves for years but say the ambushes, which once were rare and almost gentlemanly, now are frequent and more violent.

Dozens of people in the Los Angeles area, many of Colombian descent, have been arrested or questioned in connection with the heists, including a 25-year-old woman whom detectives found this spring with $2 million in stolen jewels stored in her home. They raided the site after tracking calls from a cell phone that had been mistakenly left in a jewelry salesman's car after an ambush.

But most of those in custody are refusing to talk. And the attacks have not let up. In fact, investigators believe that the growing crackdown here--the LAPD has a task force of detectives working the case--has forced the thieves to expand their operation to other cities in the West.

Law enforcement officials are also having a hard time prosecuting accused bandits because they usually work as a group and often conceal their identities with masks. In the past six months, despite the flurry of arrests, prosecutors in Los Angeles have charged only two suspects.

"These are very difficult cases to put together," said Deputy District Attorney Peter Berman. "It is very hard to pin down what kind of organization or groups may be behind this, but it is becoming pretty clear that people are being brought in from outside of the area to do a job, then they vanish."

Under pressure from police and insurance companies, jewelers across the country are trying to take more precautions. The California Jewelers Association is pleading with dealers and wholesalers to vary their driving routes and to be on the lookout for suspicious characters around jewelry auctions and shows. Some merchants even travel with suitcases filled with fake and worthless jewelry in the hope of fooling thieves if they are ambushed.

The Jewelers' Security Alliance, a trade organization based in New York, is encouraging dealers across the country to hire bodyguards, which for many independent merchants is too expensive. Barcott's security firm, which last week escorted several frightened jewelry salesmen from San Diego to Los Angeles, charges $25 per hour per officer and has a four-hour minimum for its services.

But some dealers say the extra security costs are a necessity. The alliance estimates that jewelry salesmen lost about $33 million in gems in roadside robberies last year, and that nearly one-third of the heists occurred in California and Nevada, where many large merchandise shows are held.

The recent heists occurred after many jewelry salesmen in the region had convened for a merchandise show in Las Vegas.

"It is a terrible time," said Alberta Hultman, executive director of the California Jewelers Association. "The incidents are becoming more confrontational and way more dangerous. A lot of people are saying they don't even want to risk staying in the business anymore."

"When I leave a mart, I always make sure I have someone I know following me," said Joy Cotugno, a jewelry dealer in nearby Pasadena. "You have no idea when these ruthless guys might suddenly show up and get you."

In recent months, armed bandits here have surrounded and robbed a jewelry dealer as he drove into the driveway of his home one evening. They ambushed two others as soon as they stopped for a meal at a fast-food restaurant. On the same day last week that one pair of jewelry dealers was run off a road near Los Angeles International Airport by thieves in several cars, another dealer was robbed by a group that swooped on him when he pulled into a liquor store parking lot. In each incident, the victims lost several hundred thousand dollars' worth of merchandise. Some victims have also been punched or pistol-whipped, but few have been shot.

Police say a common tactic of the gem gangs, which often use cell phones and two-way radios to coordinate a strike, is to slash a tire of a salesman's car, then pounce on him a short while later when he pulls off the road to inspect the problem. Other salesmen driving cars have been rammed and surrounded near highway entrance ramps, which allows the escaping thieves to blend quickly into Los Angeles' epic freeway traffic. The heists usually are executed so well that police say they suspect that some of the thieves are former Colombian police officers or soldiers. Very little of the jewelry is ever recovered.

"They follow their victims for days or even weeks without ever getting noticed," said Woodings, who is coordinating the police investigation into the heists here. "They seem very disciplined in what they do, but we are putting more pressure on them."

Los Angeles police are focusing their investigation on the city's bustling jewelry district downtown, where dozens of vendors sell their wares in crowded sidewalk stores and office buildings. The area also is a prime distribution site in Southern California for traveling jewelry salesmen. Investigators think the thieves often identify their targets there, so undercover and uniformed patrols have been increased. City officials also are planning to meet soon with a large group of merchants and wholesalers who do business there to discuss ways to better combat the thieves.

Faddoul Baida, a veteran downtown jeweler, said the problem has never been so dire. "It's not petty theft like it used to be," he said. "It has become a terrifying life indeed. There is no safety."

Special correspondent Cassandra Stern contributed to this report.

CAPTION: Detective Mike Woodings: "There's just a constant tide of these guys out there that we have to contend with."