A beauty magazine executive has gone underground until cops catch her ex-boyfriend - the prime suspect in a bizarre Halloween sexual assault in Chelsea.

"I want you to know that I am OK and I have to lay low," the frightened magazine exec wrote in an E-mail sent after cops began hunting for writer Peter Braunstein.

Dozens of cops and probation officers are hunting Braunstein, 41, a former writer at Women's Wear Daily who is suspected of dressing up as a firefighter and attacking a 34-year-old woman in her W. 24th St. apartment last Monday.

Meanwhile, it was revealed yesterday that Braunstein acted as if he were without a care in the world when he checked into a midtown hotel just hours after the twisted 13-hour attack.

"She's a very smart girl. The minute she heard he was going to be a suspect [in the Halloween attack] she brought in security," said a friend of the woman, whose name is being withheld by the Daily News.

"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.

Braunstein called the woman's office and her elderly father Friday and warned both "they better not talk to police," law enforcement sources said.

In the past, he has posted nude photos of the woman on the Internet, written blogs eviscerating her and flooded her with hundreds of phone calls, according to court papers and her friends.

Police now know that Braunstein checked into Room 317 at the Super 8 Hotel Times on W. 46th St. at about 11:30 a.m. last Tuesday, just hours after the Chelsea sex assault.

Braunstein left his room briefly twice, according to the electronic key records, but he didn't order food or movies, and left the room immaculate when he checked out Wednesday at 1:30 p.m., hotel staff said.

Cops suspect Braunstein dressed up as a firefighter and set two small fires to con his way into the 34-year-old woman's apartment on Halloween night.

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