One of the two alleged killers of a Jersey City, N.J. family spent nearly a week living in the same two-story building with the bodies of the family he is alleged to have slain, prosecutors said yesterday.

They said the man lived in an upstairs apartment with his wife and children as the corpses of his slain landlord’s family – bound, gagged, blindfolded with duct tape, and horribly punctured and stabbed – lay decaying one floor beneath.

The victims – Hassam Armanious, 47; his wife, Amal Garas, 37, and their daughters, Sylvia, 15, and Monica, 8 – were slain January 10, but their bodies were not found until January 15. That was when police, alerted by the wife’s brother, Ayman Garas, that the family was mysteriously out of touch, entered the residence.

Forensic experts and behavioral scientists said it is highly unusual for murderers to share the same physical space as their victims for very long – and it is even more unusual for attempted robberies to turn into violent stabbings, as Jersey City investigators are suggesting.

According to court documents, Edward McDonald, 25, a former drug dealer who was recently released from federal prison under court-ordered supervision, allegedly confessed to killing the 8-year-old daughter, Monica Armanious. He also allegedly implicated a friend he met at a federal prison at Ft. Dix, Hamilton Sanchez, who the authorities said was imprisoned for more than 10 years for conspiring to sell 3 kilos of cocaine and 1 kilo of heroin. Sanchez, 30, was living in Newark at a halfway house at the time of the killings. According to the criminal complaint filed by prosecutors, McDonald told detectives that Sanchez is responsible for killing the Armanious parents and their teenage daughter.

Prosecutors alleged that McDonald killed Monica after she broke free from the duct tape binding her arms and attempted to escape from the house. After she removed duct tape covering her eyes, she spotted her neighbor, who was wearing a ski mask over his face, recognized him, and tried to break away, prosecutors said. McDonald then went after her, prosecutors said, and Sanchez, according to McDonald’s statement to police, allegedly murdered the other three.

Federal agents picked up McDonald and Sanchez on the street at different parts of Jersey City on Thursday, and each is currently being held on $10 million bail and awaiting court hearings. A court-appointed attorney for Sanchez, John Booth, of the Hudson County Public Defender’s Office, declined to comment. An attorney for McDonald had yet to be assigned.

McDonald was one of the first possible witnesses interviewed by police detectives the night the Armanious bodies were discovered. Detectives repeatedly knocked on his door during the middle of night and, after receiving no response from anyone within the apartment, knocked down the door and found McDonald, Stephanie Torres, and their children sleeping, according to law enforcement sources. When interviewed about the case, McDonald and Ms. Torres told the detectives that they hadn’t known about the murders and that they heard no commotion going on below them at the time the murders are believed to have taken place, between 9 and 11 p.m., prosecutors said. Ms. Torres is not considered a suspect in the murders, the lead investigator on the case, the Hudson County prosecutor, Edward De Fazio, said. The investigation, he added, is continuing.

Ms. Torres could not be reached for comment. When The New York Sun asked to speak with her yesterday, a family member who did not identify herself hung up the phone.

Mr. Garas, the slain woman’s brother, said the Armanious family never spoke of having any problems with McDonald or Ms. Torres as tenants. The first time Mr. Garas met McDonald and Ms. Torres, he said, was the night the four bodies were found. Mr. Garas and his sister’s two neighbors were all waiting around the precinct house – McDonald and Ms. Torres looking for a place to stay because crime scene workers had made them leave the apartment, and Mr. Garas seeking answers in what appeared to be ritualistic murders that might be related to the Armanious family’s activities as Coptic Christians.

Still, the brother expressed doubt that McDonald could be responsible for so heinous a crime. “He had such a baby face, I never thought he could do it,” Mr. Garas said of McDonald.

“Everything is possible in this world, but this,” he said of the neighbor’s arrest, “seems like the beginning of it, not the end of it.”