It was 3 a.m. when Mario A. Rubio-Martinez, his brother and some friends pulled into the Culmore Shopping Center parking lot. They'd been to a party Saturday night at an uncle's house in Manassas and stopped to drop a passenger off near home.

They lingered for a few minutes in the Leesburg Pike parking lot, chatting about friends and work, when suddenly an unfamiliar red pickup truck packed with more than a dozen adults and children approached.

There were no introductions, just angry words exchanged. Before anyone knew what was happening, as many as 15 people piled out of the pickup, surrounded Rubio-Martinez and attacked the 22-year-old, police later said. The assault lasted less than a minute, according to one witness, and was led by three of the youngest boys. Rubio-Martinez was punched, kicked and ultimately stabbed to death.

Yesterday, a 14-year-old and two other boys, ages 12 and 13, were being held without bond in Fairfax County in what sources familiar with the case were calling a random attack by an aggressive local gang known as MS.

Police said Rubio-Martinez, who lived in Alexandria, had no ties to gangs and did not know his attackers. Police have not released the names of the juveniles who were arrested.

"They just came into the parking lot and rushed toward us," said Louis Martinez, 24, the victim's brother, who witnessed the slaying.

"They were yelling something I couldn't understand and went for Mario. . . . He tried to defend himself, but then they stabbed him and ran back to their truck" and took off.

Martinez, speaking Spanish through an interpreter, said he raced after the pickup as others tended to his brother. When he returned a few minutes later, he said, friends were cradling his brother, who had been stabbed in the upper body. He died a short time later during surgery at Inova Fairfax Hospital.

"We had never seen those [people] in our life," Martinez said. "We were just coming home and . . . not looking for any trouble."

Police found the pickup truck not far from the scene Sunday morning and arrested the three youths, all from the Culmore area. The 14-year-old faces a murder charge; the other two youths have been charged with simple assault.

Activists who work with Latino youths in Culmore say that it's not uncommon for young teenagers, especially those looking to join a gang, to come to Culmore and stir up trouble, sometimes as part of a gang initiation.

"Unfortunately, what happened is not surprising," said Iris Garay, executive director of the Northern Virginia chapter of Barrios Unidos, which is one block from where the slaying occurred. "We've seen a lot of these kinds of attacks over the last couple of years. What worries us even more is that the kids are getting younger and younger."

MS, a gang with roots in the Salvadoran community and known locally as Mara Salvatrucha, has grown in this area in recent years, Garay said.

About 25 gangs now operate in Fairfax, and gang-related violence appears to be slightly on the rise, according to one police investigator.

"Fairfax County doesn't have a very high violent crime rate," said Detective Ken Compher, of the county's gang investigations unit.

"What we're seeing now is that some of the groups that in the past were stealing cars are getting into more fights. . . . Gangs that were smaller in the past have grown and might be getting bolder."

Garay said the Culmore community has "a big problem with how to keep kids out of these gangs. Usually it doesn't lead to someone getting killed, but the younger kids are trying so hard to be like the older ones that sometimes they do just about anything."

A source familiar with the investigation said the attackers may have been looking to make a name for themselves.

"These kids were out looking for trouble," he said.

"They were little kids. It's scary."

Louis Martinez said his brother emigrated from El Salvador in 1998 and worked for a Maryland construction company.

Family members described Rubio-Martinez as a quiet and hardworking man who sent money to his parents and hoped to bring them to this area one day.