It is a rite of passage in a young Latina girl's life -- the coming-of-age party given for her 15th birthday, known as la quinceanera. There are floor-length evening dresses, chamberlains and maids of honor. Family and friends come together to honor the young woman.

It was no different for the family celebrating a quinceanera in a banquet hall at an Econo Lodge in Arlington County the evening of April 26, 2003. But it ended in chaos and death: Gang members from South Side Locos crashed the event to attack members of a rival gang, Mara Salvatrucha, who were in attendance. Cesar Antonio Rios-Garcia, 19, a guest caught up in the fight, was left lying on the floor with a fatal knife wound.

During a hearing yesterday at Arlington Circuit Court, Simon Ernesto Flores-Siliezar, 25 -- one of eight people convicted in the slaying -- was the first in the group to be sentenced under one of several new gang laws designed to stanch the violence by gangs in the state. As part of their strategy to fight an escalating gang problem, prosecutors also used an old law on the books, murder by mob, the so-called lynching statute.

Flores-Siliezar received 20 years in prison for his role in the melee -- pepper spraying the crowd at the party as fellow gang members chanted "South Side Locos." A jury found Flores-Siliezar guilty in February, and Circuit Court Judge William T. Newman followed its recommendation yesterday and sentenced Flores-Siliezar to 15 years on a murder by mob conviction, four years for criminal street gang participation and one year for unlawful wounding.

During yesterday's hearing, Raymond Kline, Flores-Siliezar's attorney, argued that his client should receive a lesser sentence because he did not stab Rios-Garcia.

"There were many people involved in this case -- a person died," Kline said. "But that person was not anywhere near Mr. Siliezar."

Kline said Flores-Siliezar was not a criminal and has only two prior misdemeanor convictions. He financially supported his mother back home in El Salvador, his lawyer said. "This is a man who deserves a second chance," Kline said. "Twenty years is too much to ask under the circumstances."

But Assistant Commonwealth's Attorney Alejandra Hammack argued that Flores-Siliezar deserved the sentence because he willingly participated in the gang attack. "He knew he was going up there for a gang fight," she said. "He knew he had pepper spray."

Hammack said one possible motive for the attack was revenge on the part of the South Side Locos for a previous machete attack on one of their members by Mara Salvatrucha.

Hammack described the chaotic scene at the Econo Lodge that evening, with attackers chanting "South Side Locos" as they entered the banquet room. As the melee broke out, she said, Flores-Siliezar "pepper sprayed a roomful of women and children."

"He showed no compassion in doing this," the prosecutor said. The attack was caught on videotape, including Rios-Garcia lying on the floor after he was stabbed, authorities said. Hammack said there was no evidence that the victim was involved with any gang.

"He was truly in the wrong place," Hammack said of the victim, adding that authorities do not know why he was stabbed.

Before he was sentenced, Flores-Siliezar told the judge, "I'm not a criminal; I'm a hard worker. That young man is dead, and I am sorry for that," he said.

Flores-Siliezar was one of three people convicted during jury trials, while the five others pleaded guilty, Hammack said. The seven others will be sentenced this summer. Prosecutors identified Eber Rodriguez, 21, as the person who stabbed Rios-Garcia. He has pleaded guilty to several charges, including murder by mob and stabbing in commission of a felony. His sentencing is scheduled for Aug. 6.

Melvin Majano, 20, and Salvador Corrado, 21, were also found guilty during jury trials. Majano's verdict was later vacated, and he pleaded guilty to voluntary manslaughter and malicious bodily injury by use of a caustic substance by a mob, Hammack said.

Corrado was found guilty in March of several charges including murder by mob and malicious bodily injury by caustic substance. Prosecutors charged that he also brandished an air gun. A jury recommended a sentence of 28 years and nine months.

Victor Menjivar, 19, and Anthony Paz-Ortiz, 21, pleaded guilty to voluntary manslaughter and criminal street gang participation.

Of the eight convicted, two were juveniles prosecuted as adults. Jose Andrade, 17, pleaded guilty to voluntary manslaughter, perjury and criminal street gang participation. Francisco Rojas, 17, pleaded guilty to murder by mob and criminal street gang participation. If he cooperates, the murder by mob charge will be reduced to involuntary manslaughter, prosecutors said.