MILTON -- Police said yesterday that Kerby Revelus had been involved in a fistfight Friday night with a next-door neighbor, a conflict that precipitated the violent knife attack on his three younger sisters in their apartment on Belvoir Street Saturday afternoon.

"We don't know what the fight was about," Milton Police Chief Richard G. Wells said yesterday. But "this all came as a result of [that] incident."

Police officers were not called to the scene of the fight, said Wells, who declined to disclose the identity of the neighbor, citing the ongoing investigation.

Police were called to 7 Belvoir St. shortly before 5 p.m. Saturday by 9-year-old Saraphina Revelus. Wells said police arrived at the home and broke down the door to a room on the second floor in time to see Kerby Revelus grab 5-year-old Bianca and slash her neck. Wells said Revelus had already fatally stabbed his 17-year-old sister Samantha "Princesse" Revelus. The officers shot and killed Revelus as he was stabbing Saraphina Revelus and saved her life.

Bianca Revelus, who was supposed to start kindergarten this fall, had turned 5 years old the day before the attack.

Friends and family struggled to understand how a brutal slaying that left two sisters stabbed to death, allegedly at the hand of their brother, could have erupted in this quiet neighborhood.

The Revelus family have lived in the house at 7 Belvoir St. for more than two decades, according to Romy Dorvil, a cousin who followed them from Haiti to the U.S in 1985. He said Kerby Revelus was moody and "always to himself" but not threatening or erratic. And he said he had never seen him behave violently toward his siblings.

"There's no way to make sense of it," Dorvil told reporters when he visited the house earlier today. "The only person who can is dead."

Norfolk County District Attorney William R. Keating said Revelus had been involved in a "domestic issue" in 2004 that involved punching but he did not elaborate. More recently, Keating said, Revelus was charged with a firearms violation that resulted in his incarceration. Keating said he was released several months ago.

Neighbors said yesterday that Kerby Revelus behaved very erratically since his release from prison.

"He would go off, just talking about random stuff, stuff that makes no sense," said neighbor Abdul Kamara. "Sometimes you would see him and he was normal. Sometimes you would see him and he was confused."

Kamara said Revelus would wander up and down Belvoir Street most mornings drinking from a bottle in a brown bag.

The house is blocked by police tape and family and friends are not allowed to go near it. Revelus's cousin Joey Dorvil walked past it yesterday morning in the rain with two friends. He said it was too early for him to talk about the situation.

"Everybody's hurting," he said. "Everybody took a loss."