A group believed to be members of a violent street gang, at least two of them wielding machetes, chased and then slashed an 18-year-old Springfield man Sunday night, an attack that Fairfax County police did not disclose until it was revealed in court documents yesterday.

The incident about 8 p.m. Sunday occurred on Augusta Drive in the Lynbrook neighborhood of Springfield, just west of Interstate 95. Police think that members of the street gang Mara Salvatrucha, or MS-13, were responsible for the attack, according to an affidavit for a search warrant served yesterday on a nearby house.

The victim told police that he saw several young men playing basketball outside 6807 Lynbrook Dr. and that one of the men flashed a hand signal indicating loyalty to MS-13. The victim told investigators from the Fairfax gang unit that he didn't want a confrontation with the men and ignored the signal.

Soon after, the victim reported, he was standing with friends at Augusta and Lynbrook drives when he saw about seven people coming toward him. Two carried machetes, and others might have had bats or sticks, the affidavit states.

The approaching group yelled "MS" and flashed more hand signals. Then one youth swung a machete at the victim's head, the affidavit states. The victim's hand was cut while blocking the swing.

The victim and his friends ran, and the victim was struck in the back of the head, suffering "extensive injury," Detective Travis Heiden wrote in the affidavit. The victim and his friends then escaped.

Police obtained a search warrant for the house at 6807 Lynbrook Dr., and officers spent several hours there yesterday, neighbors said. The gang investigators also identified a suspect who allegedly lives at the house and said he had previously admitted his membership in Mara Salvatrucha, according to the affidavit.

Fairfax police initially declined to discuss the incident yesterday, saying it was under investigation. Later, police spokeswoman Mary Ann Jennings confirmed that the attack occurred and said that the victim's injuries were not life-threatening. She said he was treated at a hospital and released. She said that a juvenile suspect was being questioned yesterday but that no charges had been filed.

Mara Salvatrucha, a Salvadoran street gang with origins in Los Angeles, is thought to be Northern Virginia's most dominant gang, with more than 2,000 members. Of 11 gang-related slayings in the region since 2000, nine are attributed to MS, as are two machete attacks in the last year in which fingers of rival gang members have been sliced off.

Several neighbors said the house on Lynbrook Drive was recently sold and then rented and had numerous occupants. They said they were dismayed to learn details of the attack four days later. "I think that they owe it to us to tell it to us," said one woman, who asked not to be identified for fear of the residents.

Lee District Supervisor T. Dana Kauffman (D) said police had not informed him of the attack. He and the neighbors said the area was not typically troubled by gang violence.

Kauffman said he has proposed that the county lobby state legislators for restrictions on machete sales. "I find it abhorrent that you can buy a machete at a shopping mall," Kauffman said.