A grieving father has spoken publicly after investigators announced new findings in the deaths of his four children, describing the case as every parent’s worst nightmare and saying those responsible should be held accountable.

On Tuesday, police in Mechanicville, New York, announced that their investigation concluded the children’s mother, Sarah Myers, and maternal grandmother, Amy Steadman, had allegedly planned the deaths of the four siblings before both women later died.

According to investigators, the case was reconstructed through forensic evidence, electronic communications and documents recovered during the investigation. Authorities allege the two women discussed how the plan would be carried out, including conversations about medications, timing and efforts to avoid raising suspicion.

Police also said investigators recovered written documents in which the authors claimed they believed they were protecting the children from alleged abuse. However, authorities stated they found no independent evidence supporting those allegations.

Police Chief William Rabbitt emphasized that regardless of the claims contained in the writings, nothing could justify the deaths of four children.

Their father, Brady Harmon, said the investigation confirmed what he had feared from the beginning.

“I knew my kids were murdered,” he told local media after learning the findings.

Harmon, who had been involved in a lengthy custody dispute with his former wife, said his children had been expected to spend part of the summer with him before their lives were cut short.

Investigators also revealed that one of the children appeared to have tried to resist during the incident, while the others are believed to have died after being given medication. Those findings were announced as part of the police investigation and have not been challenged in court because both suspected perpetrators are deceased.

Authorities said internet searches, text messages and other digital evidence helped investigators piece together the timeline leading up to the tragedy. Officers also said welfare concerns eventually led police to the home, where the bodies were discovered during a welfare check.

The case has deeply affected the local community and has drawn national attention because of the disturbing allegations surrounding the planning of the children’s deaths.

As the investigation officially comes to a close, Harmon said he hopes his children will be remembered for the lives they lived rather than the heartbreaking circumstances of their deaths, while continuing to search for answers about how such a tragedy could happen.