Luis Alberto Guerrero was no ordinary outlaw. He wore a grenade around his neck.

When his body was found last month in this border town across from Brownsville, Tex., state police said his signature grenade was still dangling over his bloody chest. A bomb squad spent hours extracting it, as well as another grenade, its pin half removed, in the clutched hand of Guerrero's dead bodyguard.

The unknown assailants who fired more than 100 bullets into Guerrero's silver Jeep on May 10 outside the popular Wild West dance hall also killed three teenage girls, leaving five corpses and two live explosives a mile from the U.S. border and shining a new spotlight on Mexico's most unusual criminal organization, known as the Zetas.

The Zetas are former Mexican army commandos who were trained to capture drug traffickers but joined them instead, around the end of the 1990s. Armed with AR-15 and AK-47 assault rifles, the 15 or so Zetas currently at large are considered the number one security threat on this busy stretch of the border.

The Zetas are accused by federal prosecutors of a wide range of crimes, from killing an estimated 100 people over the last five years and escorting millions of dollars worth of cocaine, to extorting money from small border businesses, from car junkyards to beauty parlors.

"They're more violent and have greater capacity to liquidate people," said Jose Luis Santiago Vasconcelos, the nation's top organized crime prosecutor. In an interview, Vasconcelos offered new details of this increasingly high-profile group which, he said, has expanded beyond its initial drug trafficking work: "They have started to kidnap people or extort money from them. They say, 'If you don't pay, I'll burn your business,' or 'If you don't pay, I'll kill you.' That's what they're doing now."

Apart from being from the same army battalion -- an airborne mobile unit trained in communications to track drug traffickers -- most of the Zetas were born in central Mexico; at least one is a pilot, and the oldest are in their thirties. "They're young, very young," Vasconcelos said. He said the Zetas named themselves, settling on the Greek letter zeta, which can also be Z, or "the last ones."

"They are not like other gunmen. They are well-trained and have discipline," said Jorge Chabat, an academic researcher and an expert on organized crime. Chabat said the Zetas have one other advantage: They were trained by their pursuers, the Mexican army, which is Mexico's main anti-narcotics force. While many soldiers have been accused of protecting drug cartels over the years, the Zetas appear to be the first sizable group to defect and form their own trafficking organization.

Originally there were 31 deserters, according to the Mexican attorney general's office, which has issued a special "wanted" poster for the Zetas. It bears mug shots of 31 men, many with cropped hair, who it says are dangerous and wanted for drug trafficking, homicide, kidnapping and auto theft.

Individual Zetas, like most Mexican criminals, are best known by their nicknames, which also appear on the poster. Oscar Guerrero Silva, known as "Winnie the Pooh," was found dead by federal agents in February. Another, Gustavo Gonzalez Castro, known as "El Erotica," is still being sought.

Recruited by Osiel Cardenas Guillen, whose Gulf cartel is the major operator here at the eastern end of the border, the Zetas started out as his escorts and assassins, Vasconcelos said. Cardenas himself was a former police officer who turned to the other side of the law. Despite the growing folklore around the group, including the vast sums of money the members earned from Cardenas, Vasconcelos said the former soldiers were "contaminated" by "very little money."

Shaking his head, Vasconcelos said they betrayed the army "for nothing. It's stupid. Very stupid."

What has set the Zetas apart, in addition to their superior handling of weapons and radio equipment to monitor law enforcement and rivals' activity, is their cohesion.

For instance, when the army captured Cardenas last year in a Matamoros shootout involving scores of soldiers and cartel gunmen, several Zetas with him were injured and one was believed killed. But Vasconcelos said that could not be confirmed because other Zetas risked their lives to rescue the injured. "They don't leave their wounded or their dead behind," he said.

In 2002, Zeta leader Arturo Guzman Decena, known as "Zeta 1," was killed by Mexican soldiers after he was spotted at a fast-food restaurant in Matamoros. Afterward, flowers appeared on the sidewalk outside the restaurant. According to photos published in a local newspapers, the note accompanying the flowers read: "You will always be in our hearts. From your family, The Zetas."

The army has been particularly motivated to stop the Zetas. "For the army they represent a group of traitors who must be caught and punished," Vasconcelos said. While there have been published reports that the Zetas, like some anti-narcotics agents, received training in the United States, Vasconcelos denied that.

Matamoros and the border cities of Reynosa and Nuevo Laredo constitute the home base of the Zetas, who until January were considered strictly a border security problem. Then they orchestrated a daring jailbreak in the state of Michoacan, in southwestern Mexico. Guerrero, the former soldier found dead with a grenade around his neck, and others stormed a jail in the town of Apatzingan. Wearing uniforms that resembled those of the army and federal agents, the Zetas jumped out of trucks and freed 29 prisoners, including members of the Cardenas cartel, according to the state attorney general's office.

"The jail is almost in the downtown, so it was alarming, especially for people who live around there," said Sorayda Tapia, a city employee in Apatzingan, known for its melons and mangoes. "It is usually very quiet and then to have something so surprising happen, that this big group of men shows up with high-caliber weapons, no one knew who they were or where they were from. People were really alarmed."

Guerrero, called "The Warrior," was also involved in a Matamoros jailbreak in 2001. In that incident, federal prosecutors said, he and his men outmaneuvered and outgunned 46 prison guards and freed three members of the Cardenas cartel.

The three teenage girls killed at the Wild West dance hall had been befriended by Guerrero and taken to the hall, according to interviews. Their killers have not been caught.

Francisca Morada, whose stepdaughter, Perla Lourdes Garcia, 17, was among those killed, cried as she spoke in her modest house on the outskirts of Matamoros. Morada, a former policewoman, said local authorities, who carry small-caliber weapons, were no match for the Zetas and their rivals. "It's like throwing rocks compared to what they have," she said.

Morada sobbed as she held the Mother's Day present Perla gave her the day before she died, a cheerful basket tied with a ribbon and filled with a teddy bear, flowers and balloons. "Living in this atmosphere, it's like you can't even breathe," she said. "You go out and God only knows if you'll come back."

Asked why Guerrero had not been captured even though several people interviewed in Matamoros described him as a regular at strip bars and dance clubs, Vasconcelos said federal agents get little help from local authorities as they track the Zetas.

"There is a lot of complicity on the part of the local police, and they regularly warn them, or practically help them, when we arrive to fight them," he said. "So we have two enemies there -- these guys and the local police."

Jordan reported from Mexico City. Researcher Bart Beeson contributed to this report.

Thirty-one members of the Zetas, a drug gang on the U.S.-Mexican border, are wanted on drug trafficking, homicide, kidnapping and auto theft charges.Francisca Morada holds a Mother's Day basket given to her by her stepdaughter the day before the teenager was killed in a shootout.