NY- Police have a “prime suspect” in the phony-firefighter Halloween sex attack – a deranged journalist who is already on probation for terrorizing his ex-girlfriend.

Detectives were hunting last night for Peter Braunstein, 41, a freelance writer and author who wrote about pop culture for the Village Voice and Women’s Wear Daily before his career hit the skids.

“He’s the guy we’re looking for,” a law enforcement source told The News. “Stalking, kinky sex. . . . People who know him dropped the dime on him and said we should talk to him.”

No charges have been filed against Braunstein, but cops want to question him about the Halloween horror in which a predator dressed as a firefighter drugged, molested and videotaped a Chelsea woman inside her W. 24th St. apartment.

The attacker stalked the victim, taunted her with personal details of her life during the nearly 13-hour assault and stole a pair of her high heels – a twisted trophy, sources said. The victim once worked at the same fashion magazine where Braunstein’s ex-girlfriend is a top editor, and employees there knew he had stalked his former lover, sources said. They tipped off cops, who believe a sketch of the sex fiend matches Braunstein’s description, law enforcement sources said.

Police executed a search warrant yesterday at Braunstein’s mother’s apartment in Kew Gardens, Queens, but he wasn’t there. They also visited his father’s Manhattan art gallery. “They said my son was a witness,” his father, Alberto Braunstein, told The News. “They haven’t accused him of anything. They just wanted to get in touch with him.”

“I’m not convinced in any way that he’s the one, but then every father would say the same thing,” he said, insisting his son does not resemble the sketch of the assailant.

Peter Braunstein is on probation after pleading guilty in July to menacing his ex-girlfriend.

The woman, whose name is being withheld by The News, told prosecutors he waged a demented 18-month intimidation campaign against her and her family.

“I will destroy you professionally and otherwise,” he vowed one night as he toyed with a kitchen knife after taping her hands to a chair, according to court papers.

He bombarded her with hundreds of E-mails and phone messages and sent poisonous letters to her bosses and family, including one to her elderly dad that read: “As long as I’m around, don’t expect to have another night’s sleep ever again.”

He also allegedly posted nude photos of her on an adults-only Web site, complete with an invitation for sex, plus her name, address, phone number and workplace.

After pleading guilty to a misdemeanor, Braunstein got five days’ community service and three years’ probation, court records show.

The conviction capped a period of personal setbacks and bizarre behavior by Braunstein, who did not return calls or E-mails from The News and no longer lives at the address he gave probation officials.

In 2002, Braunstein was fired by Women’s Wear Daily after he allegedly clashed with a flack for Vogue over tickets to an awards show.

Two years later, he planted an outlandish item in a gossip column, claiming his ex-girlfriend had him committed to Bellevue Hospital’s psychiatric ward. She said he cut himself and tried to blame it on her.

Still, news that detectives want to grill him about the Halloween attack stunned even those who knew that Braunstein had a dark side.

“He’s been on a downward spiral for a very long time,” one of his former lovers said. “Did I know he was crazy? Yes, that’s why I’m not in New York now. . . . Did I ever think he was capable of this? I didn’t.”

The ex-lover, like Braunstein’s father and a former editor, described him as extremely intelligent and talented, with an IQ of 185 and a charming personality.

“If it’s true,” she said, “it’s a very sad outcome for what could have been a brilliant life.”