Four people were found dead in two homes on December 30, 2025, in what investigators are calling a triple murder-suicide that spanned Mercer Island and Issaquah, Washington.
Danielle Cuvillier, 80, and her son Mackenzie P. Williams, 45, were discovered dead with gunshot wounds during a welfare check at a home in the 8400 block of Southeast 46th Street on Mercer Island. The welfare check was requested by an attorney.
Officers arrived at approximately 10:45 a.m. on Tuesday and found both individuals deceased. The King County Medical Examiner ruled Cuvillier’s death a homicide from multiple gunshot wounds to the head. Williams’ death was ruled a suicide from a gunshot wound to the head.
Investigators recovered four firearms inside the residence and a fifth firearm in a vehicle at the scene. There were no signs of forced entry at the home.
During the investigation, Mercer Island police identified concern for another individual who had previously lived at the residence. Officers requested that Issaquah police conduct a welfare check at a separate home in the 400 block of Southeast Evans Lane in Issaquah at approximately 11:40 a.m.
There, officers found two more people dead: Dominick Cuvillier, 34, and Harmony Danner, 44. Danner was Williams’ wife. Both deaths were ruled homicides by the King County Medical Examiner, with both victims having suffered multiple gunshot wounds. No signs of forced entry were found at the Issaquah home.
“The investigation continues,” Mercer Island Police Commander Jeff Magnan said, according to The Seattle Times.
The deaths followed more than a year of legal conflict between Cuvillier and Williams over the care of Williams’ disabled brother, Dominick Cuvillier, who had Angelman syndrome.
The dispute centered on caregiving arrangements for the disabled brother, who required constant supervision and assistance. The conflict escalated in January 2025 when a protection order was filed against Williams on behalf of the disabled brother.
As part of the protection order proceedings, Williams surrendered between 53 and 56 firearms to authorities. The exact number of weapons he possessed became a point of concern during the legal proceedings, highlighting the tension between family members over safety and caregiving responsibilities.
On January 16, 2025, an assault charge was filed against Cuvillier following an incident at the Mercer Island home. The confrontation occurred during a dispute over access to the disabled brother and marked a significant escalation in the family conflict.
The legal battle appeared to reach resolution when a settlement agreement was reached in August 2025. Under the terms of the agreement, arrangements were made for visitation and care responsibilities to be divided between family members.
In November 2025, Cuvillier was appointed as guardian for Dominick Cuvillier, giving her legal authority over his care and well-being. This appointment came after months of court proceedings and mediation between the family members.
The protection order that had been in place was terminated on December 10, 2025, less than three weeks before the deaths occurred. The termination suggested that the family had reached a resolution to their legal disputes through the settlement process.
The deaths have shocked the quiet residential neighborhoods where they occurred. Mercer Island, located on an island in Lake Washington between Seattle and Bellevue, is known as an affluent suburban community. Issaquah is situated in King County.
Commander Magnan confirmed that investigators are working to piece together the sequence of events that led to the four deaths. “The investigation remains ongoing,” he said, according to KOMO News.
Detectives continue to review evidence from both scenes, including conducting forensic analysis of the firearms recovered. The King County Medical Examiner has completed initial determinations on cause and manner of death for all four victims, but the investigation into the circumstances surrounding the deaths continues.
Authorities have stated there is no ongoing threat to the community. The incidents appear to be contained to the two residences and involve only the four family members who were found deceased.
The Mercer Island Police Department has not released additional details about the timeline of events or which locations were visited first by the suspected perpetrator. Investigators continue to analyze evidence from both crime scenes to establish a complete picture of what transpired on December 30.
The attorney who requested the initial welfare check has not been publicly identified.
