A jury has returned a guilty verdict in the New York murder trial of Sarra Gilbert, who allegedly stabbed her mother to death in 2016, six years after the disappearance of her sister Shannan led to the discovery of 10 sets of human remains on a Long Island beach and talk of a possible serial killer. 

An Ulster County jury Thursday found 28-year-old Sarra Gilbert guilty of second-degree murder, rejecting a defense claim she was driven by mental illness.

Prosecutors say she stabbed her 52-year-old mother, Mari, more than 200 times and bashed her head with a fire extinguisher in the mother's Catskills home last July.

Gilbert faces 25 years to life in prison when she's sentenced Aug. 4.

John Ray, an attorney who has represented the family on previous matters, said at the time of Gilbert's arrest in July that the young woman was mentally ill and hearing voices. Ray, who had not been formally retained in the murder case at the time of his comments, said she Gilbert had gone to her mother's home because she was hallucinating, and Mari Gilbert "died while trying to help her mentally ill daughter." 

Shannan Gilbert, Mari's daughter and Sarra's older sister, vanished in May 2010 after fleeing a client in Oak Beach, sparking an investigation that would lead to the nearby discoveries of 10 sets of human remains. Police believe the 10 were victims of a serial killer but Shannan Gilbert accidentally drowned in a marsh after running away from the Oak Beach home.

The medical examiner's autopsy proved inconclusive, and prior to her death, Mari Gilbert had been fighting to get Shannan Gilbert's case reopened as a homicide. Her family had also been involved in civil litigation.