Even for seasoned detectives, the crime scene in an apartment last weekend was horrific: A man, gagged and handcuffed, had been killed and his genitals mutilated.

On Monday, police were investigating whether Eric Goodridge's mentally disturbed stepdaughter lured him to her home and attacked him there to avenge a history of sexual abuse.

Investigators believe the suspect, Brigitte Harris, "did it," said a law enforcement official. "We are trying to determine why."

The official, who spoke on condition of anonymity because Harris had not been arrested or charged, said police were checking reports that Goodridge, 55, may have abused Harris as a child. Detectives were hoping to question the 26-year-old suspect at a hospital mental ward where she was admitted after the slaying, the law enforcement official said.

Police also were seeking a search warrant to comb the suspect's computer files for clues about a motive. On her MySpace.com home page, she referred to herself as The Original Dark Angel and listed her mood as "depressed."

Her attorney, Arthur Aidala, after viewing the MySpace page, said, "You could tell that she's someone who's calling out for help."

He said his client's stepfather was "a monster."

"And living with a monster from the age of 3 is something that you just don't recover from without professional help," he said.

A woman identified by the Daily News as Goodridge's former mother-in-law called him a good father.

"I would die if I could believe that," she said when asked if he had harmed Harris.

Police said it was unclear how Goodridge, who was from the West African nation of Liberia and lived in Staten Island, ended up at his stepdaughter's top-floor apartment in a three-family home in the Rockaways section of Queens. When officers were summoned there at about 1 p.m. Saturday, they discovered his mutilated body; Harris was missing.

The victim had been bound with duct tape, handcuffed and strangled before having his penis sliced off, police said. The medical examiner determined he died from "homicidal violence," including gagging and neck compression.

Police immediately began searching for Harris and learned she had checked into a hospital for psychological treatment.