NEW YORK (AP) — Sylvie Cachay, a promising New York fashion designer, had a dangerous chemistry with her playboy boyfriend, the son of an Academy Award-winning composer. They were only together six months, but they fought like crazy and made up the same way, friends say.

But on Dec. 9, 2010, Cachay's body was found in an overflowing bathtub at a members-only hotel in the Meatpacking District, among the trendiest neighborhoods in the city. She was dressed in a thick black sweater and pink and blue underwear. The faucet was on full blast. Her boyfriend, Nicholas Brooks, was absent — she was discovered by hotel workers responding to a leaky ceiling in the room below. But his DNA was on the faucet, prosecutors said.

The case was a tabloid sensation. "Death at the Soho House: Beauty in the Bath," read the New York Post cover.

Brooks, 25, is now on trial for murder after prosecutors said he strangled the 33-year-old inside the Soho House hotel room because she tried to end their tumultuous relationship. The case is expected to go until early July.

"It was an act as ruthless as it was cowardly," Assistant District Attorney Jordan Arnold said during opening statements.

Brooks has pleaded not guilty. His defense attorney Jeffrey Hoffman suggested Cachay drowned accidentally, passing out from an overdose of prescription pills she took to treat migraines and fibromyalgia, a disorder that causes widespread pain in the body. He said investigators rushed to arrest Brooks because they needed a fast suspect in the high-profile killing that became national news.

"There had to be immediate satisfaction to the press and to the publicity," Hoffman said.

The two were introduced by a friend in 2010, and were on one of their first dates when Cachay's teacup poodle Pepper was run over by a car. She had to euthanize the dog, and Brooks comforted her. The trauma bonded them, her friends said. But the two were constantly on-again, off-again. Cachay didn't like that they would drink too much together, her friends testified.

"Their relationship seemed very passionate, both in a good way and also in a bad way," her friend and personal trainer Cheri Fogelman testified. "Sometimes they were really happy together and sometimes it was really bad, angry hurtful and hard to deal with," she said.

"I think I just broke up with Nick," she texted Fogelman in late November. A few days later she said: "Nick still around."

Brooks was "a young man from a privileged background who liked to party," Arnold said. He was unemployed, dropped out of college and had a penchant for escorts and marijuana, prosecutors said. He is the son of "You Light Up my Life" composer Joseph Brooks, himself arrested in 2009 on rape charges in a casting couch scheme. The elder Brooks killed himself in 2011, his body found with a plastic bag over his head in his Manhattan apartment.

Cachay was the daughter of Peru-born parents and grew up with her brother near Arlington, Va. Her father was a physician. She studied fashion in college and worked as a designer for Marc Jacobs, Victoria's Secret and Tommy Hilfiger before she opened her own swimsuit line, Syla, in 2006. But the line struggled during the recession, and closed two years later.

Fogelman and others said the couple seemed mismatched and not only because of the nine-year age difference. Cachay was vibrant, outgoing. Brooks was brooding and often unfriendly, her friends testified. But he could also be deeply romantic, penning a sincere love letter to the designer after she threatened to break up with him.

"I want you to know that my love for you only grows more and more each day," he wrote, according to the Daily News of New York, saying he would make her the happiest "wife, mother and sister in the world" if she allowed him to prove his love to her.

Brooks had been staying with her at her Greenwich Village apartment at the time. Shortly before she died, she wrote him a five-page to-do list that ranged from the intimate to the mundane.

"Get a job," she wrote, the text of the letter read into the record for the jury by her brother. "Say sweet things."

She asked him to help out around the house, not to overuse paper towels, do something productive and take her on dates.

"No random over drinking or drug use," she wrote.

On the night of her death, Brooks knocked over a candle and started a fire at her apartment. So they went to the Soho House, an exclusive hotel and club that cost $1,800 a year and upward for a membership. Surveillance footage shows them checking in at 12:31 a.m. on Dec. 9 to Room 20, compact quarters with a Charlie Chaplin mural on the wall and an oval-shaped tub ringed by a pink curtain.

Soho House Food and Beverage Director Bryant Toth testified Friday that said the staff got a 12:53 a.m. phone call from the room for what sounded like a room service order for two: a large Cobb salad and a hangar steak. The staff was told the meal was to be eaten in the hotel's restaurant and set up a table for two in the main bar.

"How many people showed up?" Manhattan Assistant District Attorney Joel Seidemann asked.

"Just one," Toth said, and it was Brooks.

"He looked a little bit distraught. He looked a little bit angry. He had a bad day ... it appeared to me, based on mannerisms," Toth said.

Brooks ordered a Maker's Mark bourbon, picked at the food, then left. He's seen on surveillance footage coming and going from the room several times. At 2:18 a.m., he made the last trip out, putting on a coat and leaving for the night.

Cachay's body was discovered at 3 a.m.

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Associated Press writer Jennifer Peltz contributed to this report.