THE heartbroken father of four children fatally poisoned by their mom and grandmother has described the murders as a ‘horrendous, spiteful act’ as chilling new details have been revealed.

Police revealed on Tuesday that , 44, and her mother, Amy Steadman, 64, carefully plotted the killings in the hours before taking their own lives.

After a press conference, the children’s father, Brady Harmon, told The U.S. Sun that he cannot comprehend the “sick” and “sadistic” actions investigators say led to the deaths of his children.

The young victims – 10-year-old twins Gavin and Gracelynn, 11-year-old Hudson, and 13-year-old Harper – were found dead alongside Steadman and Myers in the , New York, on June 23.

Police claimed the mom and grandmother gave the children a cocktail of drugs in an  before one of them stabbed Gavin with a knife.

They allegedly killed the kids in an attempt to stop Harmon from spending the summer with them.

Investigators found a heartbreaking message from Gracelynn who was in pain and asked for help from the 𝘬iᩚller duo – who ignored her desperate plea as she battled through severe stomach pain.

They carried out the murders before taking their own lives on June 10 and the six decomposing bodies were found days later after a neighbor called for a welfare check.

Harmon exclusively told The U.S. Sun, “I cannot understand what would go through those sick, sadistic, twisted minds to carry out this horrendous, spiteful act on harming my children because I have access to them for 60 days.

“You stab Gavin. Gracelynn is trying to communicate and say, ‘I need help,’ and you guys give them nothing.

“Just more pain. More fear. That’s a father’s worst fear.”

He previously said he had spent years fighting to rebuild a relationship with his children and was days away from finally bringing them to Utah for a court-approved 60-day summer visit.

According to investigators, text messages between Myers and Steadman showed they planned to poison the children between 10:30 am and noon, hours before Myers was due to have a scheduled call with Harmon.

But when the children did not succumb to the diᩚadℓy plan as quickly as expected, one of the women sent a chilling message, “It’s not working, we need another option.”

Police said internet searches recovered from Steadman’s phone included medication reactions and the effects of insulin on non-diabetics.

Later chilling searches included, “Where’s the most fatal place to be stabbed” and “Find the carotid artery in the neck.”

One of the most haunting pieces of evidence released by police was a text allegedly sent by Gracelynn after becoming ill, reading: “Why would you do that? You both won’t let me puke.”

Later that evening, Steadman sent a further haunting message to Myers claiming, “Gracie is aware.”

Investigators believe Gavin survived  and was then stabbed to dєatᩚh with a knife, with police saying he suffered defensive wounds.

Authorities also recovered a handwritten note in which Steadman claimed the killings were intended to “protect” the children from Harmon ahead of his court-ordered parenting time.

Myers also made allegations of 𝘢bu𝘴e against Harmon, which he has denied.

Police said they found no evidence supporting the allegations made against him and he has never been charged.

The couple had been involved in a custody dispute but had reached an agreement that allowed Harmon to spend the summer with the children.

He previously told The U.S. Sun that his last FaceTime call with the kids was on June 7, when he said they appeared happy.

Harmon said that when he tried to contact Myers again days later, she told him the children were sick and they would speak the following day — but he never heard from them again.

Weeks after losing his four children, Harmon said the pain remains overwhelming.

“Even over a month later, it’s still not easier,” he said.

“And I don’t think it ever will be.”

Referencing the 𝘢bu𝘴e allegations, he claimed there was no evidence and branded Myers a “narcissist.”

He added, “I didn’t do anything.”

Harmon, who has an older daughter from a previous relationship and has since remarried, also pointed to previous concerns raised about Myers’ care of the children.

Records reviewed by CBS affiliate  showed child welfare officials investigated Myers multiple times between 2020 and 2024, with some cases resulting in findings of inadequate guardianship and educational neglect.

There is no indication that CPS found grounds to remove the children from Myers’ care.

CPS are yet to comment on the case, Mechanicville Police Chief William Rabbit said on Tuesday.

The police investigation remains ongoing as authorities continue reviewing evidence and await toxicology reports.

Following their deaths, Harmon spread their ashes on top of a mountain in Utah during a  with his family.