The sister of Shannan Gilbert, who was found dead near a suspected serial killer's mass grave, stabbed their mother to death due to her schizophrenia, according to the family attorney
Credit: Frank Eltman/AP

Mental illness drove Sarra Gilbert to kill her mother, Mari Gilbert – five years after Sarra’s sister was found dead near a suspected serial killer’s mass grave in Long Island, New York – Mari’s lawyer and others close to the family allege to PEOPLE.

“It was schizophrenia,” attorney John Ray, tells PEOPLE. “Sarra was hospitalized several times. Over the past couple of days she began to hear voices. She called her mother and said come over. … And she stabbed her to death.”

Police say Mari’s body was discovered in her daughter’s home in Ellenville on Saturday. They arrested Sarra later that day and charged her with criminal possession of a weapon and second-degree murder.

Sarra is held without bail and is set for a preliminary hearing in court on Tuesday. It is not clear if she has retained an attorney or if she has entered a plea to her charges.

“The tragedies and illness are often intertwined,” says Dottie Laster, a private investigator and missing-persons specialist who helped the Gilbert family search for Mari Gilbert’s older daughter, Shannan.

“When your loved one is missing, the torture is too much for family members. They can’t eat. They can’t sleep,” Laster tells PEOPLE. “Over time, they can develop serious medical malfunctions.”

(Local police did not immediately respond to PEOPLE’s request for comment, and have not publicly discussed a motive in the case.)

Laster says families who have a loved one go missing are often traumatized long term – and she says and it was especially so with the Gilberts. “Every phone call, every message, every knock on the door, is a combination of hope and terror,” she says. “You don’t go back to where you started… It’s like a nuclear bomb went off.”

‘All These Women Are Victims’

Mental illness had affected both Mari, 52, and Sarra, 27, according to Elizabeth Meserve, aunt of Long Island serial killer victim Megan Waterman. Meserve tells PEOPLE she became close with the Gilbert family after they were linked by the Gilgo Beach killings in Long Island.

“The problems go back a long time, when those girls were bounced around in foster homes and suffered abuse, including in their mother’s home,” Meserve says. “The truth is all these women are victims.”

After Shannan’s disappearance in May 2010, Sarra appeared at vigils and memorials alongside her mother and sister Sherre, holding photos of her missing sister.

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“They were very much on the same page, as far as trying to find Shannan,” Laster says. “They were traveling together, coordinating. Sarra seemed like she had it together. She was doing a very positive job and was helpful.”

Shannan’s body was discovered 18 months after her disappearance, on the same stretch of road where the bodies of 10 victims have been linked to a suspected Long Island serial killer, though investigators have said Shannan’s death is unconnected to the other 10.

The family “never found closure,” Laster says, as Shannan’s official cause of death remains undetermined and the Long Island serial killer remains at large.

‘She Was a Good Sister and Daughter’

Attorney Ray tells PEOPLE that since 2014, Sarra has been hospitalized in at least two facilities in upstate New York. In February, she drowned her pit bull in her bathtub and was arrested, he says, and “that case is still pending.”

“Nobody dreamed that the daughter was going to kill her mother,” he says.

PEOPLE has not been able to reach the Gilbert family for comment. But soon after Mari’s death, daughter Sherre apparently posted on Facebook about the news, according to screenshots obtained by PEOPLE.

“Yesterday was the second most devastating day of my life,” according to the post. “The first was loosing Shannan. I can t believe I m reliving this nightmare again. My mom, my best friend, the person I relied on the most in this world is gone!!!

“Mental illness is a serious disease. We tried to get Sarra help many times since she was diagnosed with schizophrenia in early 2014 and she would get better and then her condition would get worse.”

Laster says she “genuinely liked” Sarra. “I felt her determination to not just let her sister disappear,” she says. “She was a good sister and daughter. She was loyal, and I believe her illness overtook her.”

“Sarra belongs in a facility where she can get treatment, not a prison,” Meserve says. “When she has moments of lucidity, I know she’ll be devastated by what she has done.”

Laster called it “one tragedy on top of another.”

“This is a family that needed a lot of help.” Meserve says. “But they weren’t getting it.”