RENTON, Wash. (AP) _ The pistol-waving bandits shouted in Vietnamese as they burst into the house: ''Tell us where your jewelry is or we'll kill you 3/8''

Tying up the terrified Vietnamese family, they forced the children to watch as one intruder twisted their mother's leg and threatened to break it if she didn't reveal where valuables were hidden. Another bandit yanked the pants off the woman's 27-year-old niece and threatened to rape her.

This was no 1970s scene in war-torn Vietnam. It happened last year in Renton, a Seattle suburb. The victims were Vietnamese refugees, among the 1.2 million Southeast Asians who have resettled in America since 1975.

And the bandits? They and others like them have become the scourge of Vietnamese refugee neighborhoods across the United States. Banding together in violent gangs, these alienated Vietnamese youths roam the roadways coast to coast, bullying, torturing, robbing and extorting their own people.

Authorities call them ''cowboys'' for the whoop-and-holler, gun-toting home invasions that have become their trademark. The marauding gangs have existed for several years, but now they're growing bolder, more violent and more organized, experts say.

In February, five gang members stormed the Queen of Vietnamese Martyrs Parish, a Catholic church near Denver, where 30 worshipers were celebrating the Lunar New Year. The bandits demanded wallets and jewelry. When one man resisted, they shot him in the leg. The next day, five suspects were arrested speeding down a Kansas highway.

Last October, a gang from Texas rode into Lowell, Mass., where Cambodian and Vietnamese refugees now comprise at least 15 percent of the city's 100,000 residents.

''In the course of one weekend, they extorted every single Southeast Asian grocery in the city, about 15 stores. When that didn't work, they robbed them,'' said Jeffrey Davidson, a Lowell police inspector. Eight gang members were arrested.

In April, four gang members, ethnic Chinese from Vietnam, took over an electronics store in Sacramento, Calif. They demanded $4 million and free air passage to Thailand, shooting two hostages to prove they were serious. Police stormed the store; three hostages were killed by gunmen and three gang members were killed by police.

The Sacramento incident was unusual, experts said, because gang members chose a non-Asian target and seemed to be looking more for recognition than for money. But the violence did not surprise anyone familiar with the gangs.

''The Vietnamese gangs are testing the limits of American law enforcement,'' said William Cassidy, a Los Angeles-area private investigator specializing in Vietnamese crime.

Long-held fears of a Vietnamese crime syndicate are now being realized, Cassidy said.

''You've got seasoned, professional gangsters using cowboy crews nationally,'' he said. ''There are the beginnings of a Vietnamese Mafia specializing in auto theft and burglary, robbery of gold, diamond and jewelry stores, extortion, and public corruption.''

The Renton robbery, which occurred in March 1990, provides a case study of how a cowboy crew operates. Starting at their home base in San Jose, Calif., Phong Nguyen, 23, Giang Vo, 22, and Trung Le, 20, rented a shiny new Cadillac and drove overnight up the coast to Seattle, where they contacted friends.

The men staked out a Nordstrom department store until they saw two prosperous-looking Vietnamese women leave. Then they followed the women home, rushing into the garage as the women parked their car.

For nearly two hours, the bandits terrorized the family, ransacking the house and threatening to kill them if they called police. They fled with $10,000 in jewelry and cash.

The gangs choose Vietnamese families because they know refugees distrust banks and often keep their wealth at home in the form of gold and jewels. They also know refugees are reluctant to call police, who in Vietnam were often as corrupt as the criminals. And the common language and culture make it easier for gang members to intimidate their victims.

''My family was very frightened,'' said Nguyen, the robbery victim who is still too fearful of gangs to have his last name published. He moved his family from their Renton home the day after the robbery, never to return.

His case ended more happily than most. Ignoring the gang's threats, the family called police, and the next day, a Seattle detective, acting on a tip from a Vietnamese informant, arrested the bandits in the Cadillac. Three of the four men in the car were convicted of robbery and were sentenced in January to a combined 78 years in prison. The fourth was acquitted.

Though frightening, that gang's methods were relatively mild.

''We've seen people with their fingers burned to the bone,'' said San Francisco Police Sgt. Thomas Perdue. ''We've seen infants thrown against the wall until the family gives up the gold. We've seen children dipped in boiling water. Use your imagination, and it's been done.''

No firm figures exist on the number of such home invasions nationwide, but investigators agree they are on the rise - ''and we probably hear of only half of them,'' Perdue said.

Many police departments have added Asian crime specialists and stepped up efforts to build trust within Vietnamese neighborhoods. Perdue is past president of the International Association of Asian Crime Investigators, which has grown to 700 members since its founding five years ago.

Anaheim, Calif., has produced a video instructing police officers how to overcome cultural barriers. It teaches, for example, that Vietnamese who refuse to make eye contact aren't necessarily lying; they may just be showing respect.

In San Jose, Calif., where some two dozen home invasions have occurred since September, police hold monthly meetings with Vietnamese community leaders. In Lowell, Inspector Davidson visits schools to build trust among younger Southeast Asians.

The efforts are paying off. Investigators say refugees now are more willing to report crime.

The next challenge, authorities agree, will be to keep pace with the gangs, which have started diversifying. In California's Silicon Valley, a Vietnamese gang is suspected in a string of highly organized robberies of computer chips from manufacturers. Vietnamese gangs in Southern California are linking up with Latino street gangs and delving into street sales of cocaine, Cassidy said.

Also changing is the way authorities characterize gang members.

''There's the traditional stereotype of a kid with no parents, hardened by war,'' Cassidy said. ''Now you have the kids of doctors, lawyers, pharmacists and jewelers. These are kids 18 or 19 years old who have never known war. They grew up in America.''

Their sense of alienation is real, he said. Feeling neither fully American nor Vietnamese, they stick together in their wandering gangs, rebelling against their heritage, their new country and the law.

But if they expect sympathy from their fellow refugees, they're out of luck.

''We came to this country because we want to make a living. We want to live in peace like everybody else,'' said robbery victim Nguyen. ''We don't like criminals hanging around. They make shame for our community.''

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EDITOR'S NOTE - David Foster is the AP's Northwest regional reporter, based in Seattle.