WHAT A NIGHTMARE! What dark secrets were brewing inside this quiet upstate NY home before a grandmother allegedly carried out THE ULTIMATE BETRAYAL against her own family?

The grandma found dead alongside her daughter and four grandchildren in her upstate New York home appears to have poisoned them all, police said in a 𝓈𝒽𝓸𝒸𝓀𝒾𝓃𝑔 update Thursday — as the slain kids’ father spoke of his heartbreak at never seeing them again.

A handwritten note found at Amy Steadman’s home “strongly suggests” that she was “involved” in the deaths of her daughter, Sarah Myers, 44, and Myers’ four children: Harper Harmon, 13, Hudson Harmon, 11, and 10-year-old twins Gavin and Gracelynn, Mechanicville Chief of Police Bill Rabbitt told a press conference.

One of the children was stabbed, while the rest appear to have been poisoned — possibly in a rage over their dad recently getting custody, sources told Channel 13.

Sarah Myers and her four children (pictured), along with Myers’ mother, were all found dead inside the Mechanicville apartment Tuesday evening.Facebook / Sarah Myers

“There is recovered evidence from inside the apartment indicating intentional poisoning – to include numerous prescriptions and over-the-counter medications,” Rabbit told reporters, noting no evidence of anyone else being involved.

“Evidence recovered during the investigation, including a handwritten note, and other circumstantial evidence, strongly suggests that Amy Steadman was involved in the deaths,” he said, stressing that the investigation is ongoing.

Sources told Channel 13 that the children were injected with drugs, with syringes found inside their grandma’s home.

The relatives had all been dead “for an extended period of time” by Tuesday evening, when a concerned neighbor who hadn’t seen Steadman in several days called police for a welfare check around 6:20 p.m., according to Rabbitt.

The bodies were too badly decomposed for responding officers to immediately identify them, he said.

Steadman had allegedly grown resentful after her grandchildren’s father, Brady Harmon, was recently granted two-month custody of the kids — which was due to start next Wednesday, sources told Channel 13 News.

“Evidence recovered during the investigation, including a hand-written note, and other circumstantial evidence, strongly suggests that Amy Steadman was involved in the deaths,” Rabbitt said.Facebook / Amy Steadman

The dad, who hasn’t seen the kids since 2019, was excitedly preparing for their arrival at his Utah home until cops knocked on his door to break the harrowing news Tuesday, he told the Times Union.

“I went from, ‘I’m seeing my kids,’ to ‘I’ll never see my kids again,’” Harmon said, according to the outlet. “Hope to hell.”

“Things happen, but nothing like this.”

Harmon accused his slain ex-wife of keeping the kids from him, and only allowing for an occasional FaceTime.

The relatives had all been dead inside the apartment “for an extended period of time” by Tuesday evening, when a neighbor called police for a welfare check around 6:20 p.m., Rabbitt said. CINDY SCHULTZ

He now wants to bury his children in Utah.

“I want them as far away as possible from their mother,” he told the Times Union between tears.

The police chief said the investigation remains ongoing and will take “a considerable amount of time to get right,” according to Rabbitt.

But he stressed that investigators “believe these deaths occurred entirely within the apartment and found no evidence indicating the involvement of any outside individual.”

“Final determinations regarding responsibility will be made only after all investigative findings, toxicology results and medical reviews have been completed,” Rabbitt said.