A federal jury in Alexandria yesterday deadlocked on a death sentence for two members of the violent street gang Mara Salvatrucha, meaning that the two men will spend the rest of their lives in prison for the killing of a 17-year-old government witness.

Under federal law, Ismael Cisneros, 26, and Oscar A. Grande, 22, will automatically be sentenced to life in prison without parole for stabbing to death Brenda Paz, who was 16 weeks pregnant with a boy when her body was found by a fisherman on the banks of the Shenandoah River in July 2003.

After the verdicts were read, both men hugged their attorneys and smiled as they were led by U.S. marshals from the heavily guarded courtroom. Outside, Grande's sister said both families were relieved that the men would be spared.

"We're just thanking God right now," said Flor Grande Flores, 24, standing among relatives of both defendants. She wiped tears from her face and added, "We prayed so hard for them . . . and we will continue praying to God for them. We know God makes miracles."

The sentencing verdicts capped a two-month, high-profile trial that shed light on the inner workings of the gang, known also as MS-13, which has been responsible for a number of killings, shootings and assaults in the Washington region.

Prosecutors had argued for the death penalty, saying that Paz's "senseless slaughter" undermined the judicial system. Still, the verdict marked what they called a significant step in the federal crackdown on MS-13, considered the largest and most violent street gang in Northern Virginia. Federal prosecutors have been investigating MS-13 for several years, and law enforcement sources have said the goal is to cripple the gang by targeting its leaders.

"These murderers will spend the rest of their lives behind bars, and gang members now know the murder of witnesses will be severely punished," U.S. Attorney Paul J. McNulty said in a statement.

Attorneys for Cisneros and Grande said they were relieved that the jury could not agree on the death penalty.

"I'm really glad that we work in a profession and with a system where we can save even the most insignificant lives," said David Baugh, one of two lawyers who represented Grande.

Nina Ginsburg, one of two attorneys for Cisneros, added that the defendants and Paz all led sad lives that were entrenched in the culture of the gang.

"You can't grow up with the kind of values that the rest of us have when you spend your whole childhood being attacked by your father. I'm very grateful that the jury understood that people who grow up with that kind of script can't function the way we function," she said.

Among the factors the jury considered were arguments that Cisneros and Grande were victims of impoverished, abusive upbringings and that those same circumstances led them to join a pathological gang that became their support system.

Cisneros's attorneys said he was beaten repeatedly by his father, suffered some brain damage and might be mentally retarded.

In Alexandria, a federal jury has never voted for a death sentence, despite a jury pool widely regarded as conservative and pro-government.

Since 2001, federal juries nationwide have declined to sentence defendants to death in 64 of the 81 cases in which prosecutors have sought capital punishment. After 13 consecutive rejections over the past six months, a federal jury in Chicago sentenced a podiatrist to death late last month for killing a witness in a Medicaid fraud prosecution.

Experts attribute the trend to several factors, including declining violent crime rates and the exoneration of some death row inmates from DNA and other evidence.

Paz was killed because she cooperated in investigations of gangs in Virginia and across the country, according to testimony. She was killed weeks after she left the federal Witness Protection Program, and authorities mounted a furious effort to arrest her killers.

The trial began April 11 with four defendants. Two of them -- Denis Rivera, 21, and Oscar Garcia-Orellana, 32 -- were acquitted. In finding Rivera not guilty, the jurors rejected a central piece of the prosecution's case: that Rivera orchestrated the killing while jailed in Arlington and Fairfax counties. Rivera is serving a life sentence for another gang killing.

During the guilt phase of the trial, prosecutors called almost 50 witnesses, many of them former MS-13 members. They recalled raucous parties -- some at an FBI safe house in Silver Spring where Paz was staying -- along with crimes ranging from vandalism to gang rapes.

Prosecutors said the gang issued a "green light" on Paz, code for an order to kill, after discovering that she was working with investigators in at least six states. A runaway from Los Angeles and Texas, Paz was known in the gang as "Smiley" for her wide smile and bubbly personality.

The final plan, prosecutors said, was hatched during a meeting at the Holiday Inn at Fair Oaks Mall the night before Paz's killing. Prosecutors said the defendants repeatedly stabbed Paz with two knives after luring her to the Shenandoah on the pretense of a fishing trip.

Gang graffiti appears on a Shenandoah River bridge near where the body of Brenda Paz was found. MS-13 members lured her there for a fishing trip.