The U.S. marshals who run the federal witness protection program have one of the most daring and difficult jobs in law enforcement: protecting witnesses who put their lives at risk by cooperating with the government.

Especially when those witnesses no longer want to be protected. Case in point: Brenda Paz.

Paz, a key informant in the crackdown on the violent Mara Salvatrucha street gang, or MS-13, left witness protection after she was unable to resist the lure of gang life. Her body, repeatedly stabbed, was found on the banks of the Shenandoah River in July 2003. She was 16 weeks pregnant.

Two MS-13 members were recently convicted in U.S. District Court in Alexandria of killing Paz, 17, while two other members were acquitted. On Thursday, jurors began debating whether to sentence the two convicted men -- Ismael J. Cisneros, 26, and Oscar A. Grande, 25 -- to death. They will head back to the jury room tomorrow.

In the courtroom, defense lawyers tried to put the witness protection program on trial as much as the defendants.

The lawyers cited a psychological report saying Paz would require "close monitoring" in witness protection because of her age and isolation. She did not get that supervision, they argued, even though authorities knew from the start that Paz would be trouble.

When Paz first became a government informant, she was placed in an FBI safe house in Silver Spring. Testimony revealed that it became a hangout for MS-13 and the scene of drinking and drug use. When Paz became a witness in a case federal prosecutors were preparing against MS-13 in Alexandria, she formally entered the witness protection program in March 2003.

A government report cited by the defense in court showed that Paz soon had the same problems.

She was asked to leave a hotel in Philadelphia because of loud partying and guests in her room. Shuttled to Kansas City and Minnesota, Paz admitted to her supervisors that she was breaching security and staying in touch with MS-13, the report said.

During Paz's 31/2 months in witness protection, the defense said, the paperwork supporting her new identity was not even processed -- leaving her unable to work or enroll in school.

"They knew that putting her alone in a hotel was a recipe for disaster, and they kept doing it, knowing that she had all these problems,'' said Nina Ginsberg, an attorney for Cisneros. "I find that really blameworthy.''

Dave Turner, a U.S. Marshals Service spokesman, said he could not comment on Paz's case because the testimony given by a marshal's inspector remains under court seal. He said that Paz's death was "felt very deeply" and that the agency is conducting a "very heartfelt review" of its procedures in response.

Paul J. McNulty, the U.S. attorney in Alexandria, said the criticism of Paz's stint in witness protection was "untrue. They did all they could to help her.''

"There is only so much the government can do to protect someone,'' McNulty said. "Obviously, the individual being protected has to buy into it.''

Gerald Shur, who founded the witness protection program for the Department of Justice, echoed that point. "It is imperative that the witness cut their ties,'' he said. "You tell them what you can to help them stay alive and hope they follow the rules.''

Federal statistics show that since the program started in 1971 as a way to fight organized crime, the marshals have protected 7,700 witnesses and 9,800 of their family members, giving them new identities and new lives.

No witness who has followed the rules has ever been killed, federal officials said.