A woman whose daughter’s disappearance in 2010 ultimately led the authorities to unearth 11 bodies — her daughter’s and 10 others — along a desolate seaside highway on Long Island, was found dead on Saturday in upstate New York, and her younger daughter was charged with killing her, the police said.

[Gilgo Beach Murders: Police reveal clue handled by serial killer suspect.]

Mari Gilbert, 52, was found dead around 2:15 p.m. in the Ellenville apartment of her daughter Sarra Elizabeth Gilbert, according to the Ellenville Police Department. The younger Ms. Gilbert, 27, was charged with second-degree murder and fourth-degree criminal possession of a weapon, the police said. She was being held without bail in the Ulster County Jail and was scheduled to appear in court on Tuesday, the police said.

Mari Gilbert, who also lived in Ellenville, had been trying for years to solve the mystery of what happened to another daughter, Shannan Gilbert of Jersey City, an aspiring actress who also worked as a prostitute and was 24 when she disappeared in May 2010. While searching for her, the police found the remains of 10 people — eight women, a man and a toddler — mostly along Ocean Parkway.

Image Shannan Gilbert Credit... Jersey City Police Department, via Associated Press

Shannan Gilbert’s remains were subsequently found off Gilgo Beach in Babylon in December 2011. Suffolk County police officials have said they believe Ms. Gilbert wandered aimlessly after leaving a client’s home and wound up in the marshy area, while the others may have been the victims of a serial killer. None of the murders have been solved, and the police asked the Federal Bureau of Investigation for help last year.

Mari Gilbert, upset over the police’s inability to catch whoever killed her daughter, had hired the lawyers John Ray and Vesselin Mitev. “My attorney is going to solve this case,” she said at a funeral service for her daughter in March 2015 in Amityville, N.Y.

On Sunday, Mr. Mitev called Mari Gilbert’s death a “most morbid turn in this already complete tragedy.”

“There’s just no way anybody could’ve expected this,” he said.

Mr. Mitev said he planned to “forge forward and seek justice on Shannan’s behalf,” using a court-appointed representative to represent the estate in the case. He said that most recently he had been trying to get the police to release 911 call records from the day Shannan Gilbert disappeared.