Violence and gang life were wearing on Brenda Paz, and she was looking for a way out.

As the daughter of a member of the notorious Mara Salvatrucha gang, she knew nothing but the streets. When Arlington police picked her up in a car theft investigation last summer, she saw the opportunity for a new start.

"Smiley," as she was known in the gang for her charm and cheerfulness, soon was telling detectives from Arlington, Fairfax and Alexandria -- and Texas, North Carolina, California, Colorado and Idaho -- about stabbings, shootings and armed robberies. Her information was crucial to more than a half-dozen investigations. including a federal probe that sought her encyclopedic knowledge of Mara Salvatrucha: Structure, nicknames, license plates, she knew it all. In exchange, she was promised a place in the federal Witness Protection Program, all before her 18th birthday.

Brenda Paz had a new name, was taking classes and had the promise of a new beginning.

"For a while," said Greg Hunter, an Arlington lawyer and Paz's court-appointed guardian, "this had a happy ending."

But authorities said the lure of her gang friends and the freedom of following no rules proved too strong. Bored and unhappy, Paz left the Witness Protection Program in June after just a few months. Barely three weeks later, on July 17, a fisherman found her tattoo-covered body on the banks of the Shenandoah River. Paz, 18, had several stab wounds. She was 17 weeks pregnant.

Members of Mara Salvatrucha, or MS-13, are suspected in her death.

"She was a mess. She just couldn't be alone," Hunter said of Paz's life in witness protection. "Was it hard for her to stay away from the gang? No. Was it hard for her to stay away from the people? Yes. She'd give the gang up, but not some of the people.

"She never had to follow the rules before. She was MS."

Brenda Paz's story, pieced together from hundreds of pages of court documents and interviews with law enforcement officials and others who knew her, is one of an intelligent, vivacious girl who had no family to rely on besides one of America's most violent street gangs. She was smart enough, even as a teenager, to know it was time to move on, but she never had the grounding to quite go all the way.

"Her life was pretty much gang Mecca," said one Northern Virginia investigator who got to know her well. "All her contacts, all her friends, all her stories." At the same time, the investigator said, she was tired of gang life. "She showed signs of, 'Hey, guys, this is getting old.' She desperately wanted to get out."

Paz's death saddened law enforcement officials throughout the region, where she had become an invaluable witness against Mara Salvatrucha. Police universally point to MS-13 as the most dangerous and fastest-growing street gang in Northern Virginia, and members have been linked to a number of killings in recent years.

Authorities say Paz offered a unique knowledge of the gang's history and structure. She knew the gang's leaders and their nicknames and had an uncanny ability to take investigators inside the minds of its members.

"There were times when she would say, 'Why don't you all do this?' " said one investigator. "I would say, 'That might not work.' And she'd say, 'No, you need to think like them. Do this. Go there.' And it would make sense."

That innate intelligence struck those who met her.

"If she had been born into a white, suburban family, she would have been president of her chess club at Wellesley," said Hunter, who tried desperately to persuade Paz to stay in witness protection. "She was an amazingly intelligent and charming young woman who read Dostoevski and Cervantes."

But the news of her death was not a complete surprise to authorities, who had heard that she might be in danger and say Paz was oddly naive about the risks she was taking.

"She had zero understanding of the personal threat to her," said Sgt. Alan Patton of the Grand Prairie, Tex., police department, who along with a colleague interviewed Paz in Fairfax County in September about a 2001 homicide in Grand Prairie. Paz provided key details about the killing of a 21-year-old man who was slain by MS-13 members while she was living in Texas.

"We left there and both felt Brenda would end up dead by the hands of MS-13," Patton said. "I left there feeling sorry for her in a respect. She said she wanted to go to school, get her diploma. But I didn't get the impression she was ever going to get any of that done. It's predictable, but sad all the same."

The loss of Paz was a particular blow to federal officials in Alexandria, where sources say she was expected to be a government witness in a key homicide investigation. Three alleged MS-13 members, including a man described in court papers as the leader of MS-13 in Northern Virginia, are scheduled to go on trial next month in the September 2001 death of Joaquim Diaz, 19, who was stabbed to death on federal property on Daingerfield Island in Alexandria along the George Washington Parkway. All three have pleaded not guilty.

The Diaz slaying helped trigger a broader federal grand jury probe of MS-13. Law enforcement sources describe the investigation as far-reaching and say it is expected to extend beyond the Diaz case.

"There's a general recognition that the gang problem has grown more serious in terms of the threat it poses to the larger community," said U.S. Attorney Paul J. McNulty, who is heading the probe. He declined to discuss Paz's role in the investigation.

Diaz, of Alexandria, also was found by a fisherman with multiple stab wounds to his heart, torso and hands. His throat was cut from one end to the other, and a portion of his larynx was removed and found near his body. Authorities say in court papers that he was killed because he was affiliated with a rival gang.

Such violence is typical of MS-13, say law enforcement officials, who have been tracking the gang ever since it was founded in Los Angeles in the mid-1980s by young men from El Salvador. Many of them immigrated with military skills honed from serving on death squads at home, and the gang's founders quickly became feared around Los Angeles.

Paz was born in Honduras but was raised on those Los Angeles streets. Little could be determined about her parents. But at 12, she followed in her father's footsteps and was initiated into MS-13. Some girls are "sexed in" to the gang by having sex with a member. Paz was "jumped in," which required that she be beaten.

Mara Salvatrucha became Paz's life, law enforcement officials say. She dropped out of school in eighth grade and hung out with members in Los Angeles and, a few years later, traveled with them to Texas. Paz dated high-ranking gang leaders. "They would go from place to place because they either knew somebody there or it was an area where they could hang out or find somebody to take them in," one investigator said.

Patton, the Grand Prairie police sergeant, said that when detectives there first questioned Paz about the December 2001 killing -- while she still lived in Texas and before she began cooperating with investigators -- they got nowhere. "She very much had the gang-banger attitude," he said. " 'I wasn't there. I don't know what you're talking about. I have nothing to say.' "

Sometime last year, Paz followed gang members to Virginia. She was quickly accepted into local MS-13 culture -- "she had her bona fides" from Los Angeles and Dallas, one investigator said.

When she was arrested in Arlington in June 2002, only a few months after her arrival, she was with her boyfriend, Denis Rivera, one of the highest-ranking MS-13 members in Northern Virginia, law enforcement officials said. Rivera is now charged in the Daingerfield Island slaying and has a history of violent crime in the region, according to court records.

The Arlington courts immediately appointed Hunter her guardian because she was a juvenile and had no relatives in the area. Fairfax police soon picked her up on a warrant charging that she was an accessory to a shooting that involved Rivera.

Hunter was nice to her. The Fairfax detectives were nice. And she started talking.

"She finally had people who were listening, people who cared about her," Hunter said. "Personal loyalty was everything to her. There were police officers who were good to her, and for that she was being good to them. She had never had a legitimate adult be supportive of her before. The gang is about the gang and the leaders who, through their bravado and brutality, control it."

Even investigators were surprised by the level of Paz's cooperation. "I said, 'Why are you doing all this?' " one investigator said. " 'These are your people, the people you adore. This is your family.' She said she felt it's a way out. It's a way to start turning the corner. She would say, 'Well, there's only two ways out of the gang for me. Either I help you put these people away and I don't have anyone to hang out with and I'm forced to find new friends. Or they kill me.' "

Paz had plenty of information to share. Detectives from across Northern Virginia came to speak to her. Soon, members of a regional gang task force, including an FBI agent, began questioning her. She helped the federal agents and she helped detectives from other states where MS-13 is prevalent.

Last September, Patton said, he and a detective flew to Virginia to see Paz.

She had been in Grand Prairie the night the 21-year-old was shot, and she told them how the man was threatened at gunpoint and was made to pull his pants down. Paz described how gang members cleaned their fingerprints off the victim's car, and she said one gang member took the victim's shoes. Her statement helped prosecutors secure guilty pleas in the case.

"She had a memory like you couldn't believe," Patton said. "It caused you to wonder if she hadn't embellished a little, but the details that mattered to us were confirmed."

On the streets, word spread that Paz had betrayed MS-13. Authorities learned of threats against her. Eventually, she was "green-lighted" by gang leaders, code for an order to kill, according to law enforcement authorities.

Hunter and an FBI agent worked to protect Paz, and in November, she was released from juvenile detention in Fairfax and went into what law enforcement authorities called a safe house. But Paz had trouble adjusting to the rules and missed her social life. So she left.

"She had walked away for a party with some MS members," Patton said.

One investigator said she was playing both sides. "She was still hanging with the gang," he said. "Here she was, trying to portray to us that she was trying to change her life, but when we turn our eyes, she was still hanging. She went back. She never really left it. That's the part that angered me. She appeared to be trying, but was she really trying? The answer is no."

As the winter wore on, gang detectives learned that the danger to Paz was increasing. Local and federal authorities, along with Hunter, tried to find her. Finally, she recognized the danger and contacted them.

With the help of Hunter and the FBI agent from the regional task force, Paz entered the Witness Protection Program in March, law enforcement officials said. Now she had a new name. She was moved to a new location. She told friends and investigators that she was pregnant -- and very happy and excited about it. She was taking classes and wanted to go to college.

But something, maybe the rules, maybe the loneliness, made her walk away.

Hunter said Paz called him after she left the program. He said she knew she was in danger, but she thought she could handle the threat. She'd always been able to get out of trouble before.

"She believed up until the time she died that she could talk her way out of it," Hunter said.

Staff writers Patricia Davis, Tom Jackman and Elaine Rivera and researcher Bobbye Pratt contributed to this report.

Brenda Paz was born in Honduras and grew up with the Mara Salvatrucha gang in Los Angeles and Texas before moving to Northern Virginia last year. She died at age 18.Brenda Paz's body was found in Shenandoah County about three weeks after she left the federal Witness Protection Program.