The heartbroken father of four children killed by their mother and grandmother in a horrifying upstate New York murder-suicide said it was “sickening” to know how they were put through “such unimaginable” suffering.

Brady Harmon revealed his pain as he shared a police update showing that his kids — Harper, 13, Hudson, 11, and 10-year-old twins Gavin and Gracelynn Harmon — were poisoned with a mixture of allergy meds and a key ingredient in so-called gas-station heroin.

“I will never understand how two people who claimed to love and care about these children could deliberately cause them such unimaginable harm,” he said of his ex-wife, Sarah Myers, 44, and her mother, Amy Steadman, 64, who killed the kids and then themselves, according to cops.

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“The pain is still as raw as if it happened yesterday,” he wrote on Facebook three months after the shocking mᴀss murder and suicide in Mechanicville, New York on June 10.

“Reading today’s official toxicology findings makes it even more sickening to understand what was done to them.”