By Associated Press
Updated July 11, 2013 at 11:00 PM EDT

The playboy boyfriend of a fashion designer was convicted Thursday of strangling and drowning her in the bathtub of a swank hotel room after a tumultuous six-month relationship.

Nicholas Brooks, whose father was an Oscar-winning composer who wrote “You Light Up My Life,” put his face in his hands silently as the murder verdict was read. His girlfriend, Sylvie Cachay, was found partially clothed in an overflowing tub on Dec. 9, 2010.

Cachay’s friends cheered and Brooks’ sister wept loudly and shook as the judge set a sentencing date of Aug. 26.

“Today justice was done, but the pain is immense, and it will never go away,” Cachay’s mother, Sylvia Cachay, said in Spanish. “But she had a precious spirit, and that will live on. She’s with me always. She was with me in the courtroom, and she is with me now.”

Brooks, 27, faces a possible sentence of 25 years to life in prison.

Defense attorney Jeffrey Hoffman argued 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 suspect in the high-profile killing.

He said during closing arguments this week that Cachay, 33, was “under the influence of drugs, partially clothed, to say the least not in a rational state” when she got into the tub. Outside court Thursday, he said he was “very disappointed” and would appeal.

Prosecutors sought to show that Brooks strangled Cachay because she was breaking up with him. The medical examiner ruled that forcible drowning and strangulation caused her death, in part because of bruising on her neck and burst blood vessels in and around her eyes.

Assistant District Attorney Joel Seidemann said there was obvious tenderness between Brooks and Cachay but their relationship was made up of extreme highs and lows — and Brooks killed her in a low.

“When he held her neck and dunked her underwater … that was the defining moment,” he said during closings.

Jurors deliberated over three days, requesting nearly every piece of evidence in the trial, and haggling over suggestions Hoffman made linking the possibility of Cachay’s death to some type of rough sex, even though the judge refused to allow specific testimony on the topic at the five-weeklong trial.

Outside court, one juror said he felt the case was unfair. Hoffman said several jurors teared up during the verdict.

“I could see the frustration on their faces,” he said.

Cachay’s parents said the verdict would send a message to women around the world who are abused — they can and should get help.

“She was a beautiful girl, and she fell into a relationship and she was abused,” Cachay’s mother said.

Brooks and Cachay were introduced by a friend in 2010 but were constantly on-again, off-again. Cachay didn’t like that they would drink too much together, her friends testified. She wanted Brooks to get a job and be more proactive about the relationship.

Cachay worked as a designer for Marc Jacobs, Victoria’s Secret and Tommy Hilfiger and had her own swimsuit line.

Brooks was a college dropout who worked odd jobs, and prosecutors said he had a penchant for escorts and marijuana. His father, Joseph Brooks, who won the Academy Award for best original song for the Debby Boone ballad “You Light Up My Life” in 1977 and wrote and directed the romantic comedy of the same name, killed himself in 2011 after he was charged with sexual assault in a casting couch scheme.

On the night of Cachay’s death, Nicholas Brooks knocked over a candle and started a small fire at her apartment, burning her hair. So they went to the Soho House, an exclusive Manhattan 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. Brooks is then seen pacing the hallway, then going down to the lobby and returning several times before he puts on a coat and leaves at 2:18 a.m.

Cachay’s body was discovered by hotel staff at 3 a.m.