On December 30, 2025, Harmony Danner, a 44-year-old woman and physical therapist from Mercer Island, Washington, was found dead in her Issaquah home. She, along with her brother-in-law, Dominick "Nick" Cuvillier, 34, were victims of a tragic double murder-homicide that spanned two cities and claimed the lives of four family members. The King County Medical Examiner's Office later confirmed that Danner died from multiple gunshot wounds.
The investigation into Danner's death began after her husband, 45-year-old Mackenzie Williams, sent a concerning email to friends about his gun collection, which prompted them to request a welfare check. Around 10:45 a.m., police discovered the bodies of Williams and his 80-year-old mother, Danielle Cuvillier, at her Mercer Island home in what appeared to be a murder-suicide. This discovery led police to conduct a welfare check at Williams and Danner's residence in Issaquah, where they found the bodies of Harmony Danner and Dominick Cuvillier.
Investigators believe that Mackenzie Williams is the sole suspect in the killings. It is believed that he first shot and killed his wife, Harmony, and his brother, Dominick, in their Issaquah home. He then drove to his mother's house on Mercer Island, killed her, and then took his own life. Williams died from a self-inflicted gunshot wound.
The motive behind the tragic events appears to stem from a contentious and year-long family dispute over the care of Dominick, who had a rare genetic disorder called Angelman syndrome and required significant daily assistance. Court records revealed a history of conflict between Mackenzie Williams and his mother, Danielle Cuvillier, regarding Dominick's guardianship. This included a previous physical altercation and a temporary protection order that required Williams to surrender 53 firearms to the Issaquah Police Department. Despite the ongoing tensions, there are no outstanding suspects, and the case has been classified as a triple murder-suicide.
