Brenda Paz's life was in danger. When the 17-year-old was arrested by Virginia police in June 2002, she unexpectedly started telling them vivid tales about life as a member of the violent Mara Salvatrucha street gang, better known as MS-13. Her boyfriend was a gang leader and a murder suspect. Paz knew MS-13's cardinal rule--talk to the cops and die--but she hated rules, and loved to talk, and the police were very good listeners. A Honduran-born runaway who was raised in Los Angeles, she joined MS-13 at 12 and witnessed dozens of crimes, including murders. Paz's memory was so vivid that the Feds enrolled her in the witness-protection program to keep their new informant safe from fellow gang members. "She wasn't just a witness," Greg Hunter, her court-appointed lawyer, told NEWSWEEK. "She was like the Rain Man of witnesses."

Paz was relocated to another state and furnished with a new name and Social Security number. She was warned to be inconspicuous and to avoid any contact with gang members. But Paz chafed under the rules. She called old friends and invited some to visit her. Then, in June 2003, desperately lonely and homesick, she fled her safe house and returned to northern Virginia. A few days later, two fishermen found Paz's stabbed, bloated body on a riverbank.

Four MS-13 members are now on trial for her murder. The case has helped illuminate the secretive world of the witness-protection program, and in particular the storied agency's latest challenge: learning to protect a new generation of witnesses who, like Paz, are younger, less disciplined and more likely to ignore the rigid rules that keep them safe. Lean budgets and a bureaucracy set in its decades-old ways have constrained the program's ability to adapt. Intimidation and violence against witnesses have risen sharply in recent years, increasing the agency's workload. The program now protects more than 17,000 people, up 12 percent from 1995. Yet at the same time, the number of agents who handle cases has declined by nearly 30 percent. A recent Justice Department audit expressed "serious concerns" about morale at the agency. The program's first chief, Gerald Shur, says he fears it has "lost some of its luster."

Created during the government's 1960s campaign against the Mafia, the program has been immortalized in scores of gangster movies. Its clients included mafiosi like Sammy (The Bull) Gravano, the hit man whose testimony helped bring down John Gotti. The '80s and '90s were filled with witnesses from drug-cartel cases. These days, increasing numbers of witnesses to street-gang crimes are seeking protection. "Suddenly we were seeing a kind of defendant that we hadn't seen before," says a former federal prosecutor.

In one recent case, a teenage girl was admitted to the program after witnessing two murders. But she returned to her old haunts despite the danger. She was relocated four times and sent to drug rehab, where she stole other patients' phones to place calls to gang friends. "She was a fatality waiting to happen," says one former inspector. Prosecutors videotaped her testimony in case she was killed. She eventually abandoned the program.

Brenda Paz was just as difficult to protect. Once in the program, she got pregnant and twice left her new home in Kansas City, Mo., to reunite with gang members. By May 2003, she was living in an Embassy Suites hotel in St. Paul, Minn. Yet she was hardly keeping a low profile. Two carloads of MS-13 members came to visit her. They took turns soaking in the hot tub, and hid in the bathroom when Paz's "stepfather" (the handler assigned to her) stopped by to check on her. When they returned home, Paz left with them. Within days she was dead.

Always secretive, the agency declined to comment about Paz, or give details of how it plans to change to meet the needs of its new, volatile charges. "We're constantly critiquing ourselves," says one official. But no matter how much the agency reforms itself, it may never be able to protect witnesses who refuse to protect themselves.