Four people were found dead in Mercer Island and Issaquah, Washington, on Wednesday morning in what authorities are investigating as a triple murder-suicide that followed more than a year of bitter family disputes over the care of a disabled adult son.
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 around 10:45 am. Officers responded after an attorney representing Cuvillier reported receiving a concerning email that raised fears for the occupants’ safety.
“The investigation remains ongoing,” Police Commander Jeff Magnan said. Police found no signs of forced entry at the residence.
Officers recovered four firearms inside the Mercer Island home and discovered a fifth firearm in a vehicle at the property. The Mercer Island deaths are being investigated as a homicide and suicide, though authorities have not identified which person fired the shots.
After discovering the bodies at the Mercer Island location, detectives asked Issaquah police to conduct a welfare check at a residence in the 400 block of Southeast Evans Lane in Issaquah. Officers arrived at that location and found two additional people dead, both with gunshot wounds. The victims included Dominick Cuvillier, 34, Danielle’s younger son, and Williams’ wife.
The four deaths occurred after an extended guardianship dispute centered on Dominick, who was diagnosed with Angelman syndrome. This rare genetic disorder causes severe developmental delays and impaired coordination, requiring individuals to receive nearly constant supervision and assistance with basic daily tasks including feeding, hygiene and mobility.
Over time, Williams grew increasingly concerned about his brother’s care while living with their mother, documenting in King County court declarations that he witnessed physical and emotional abuse. Cuvillier denied these allegations.
In January 2025, nearly a year before the deaths, Williams met with an attorney and worked with Dominick to create a new power of attorney document that would transfer responsibility for Dominick’s affairs to Williams. Cuvillier responded by claiming her younger son had been unduly influenced to execute the new power of attorney.
Tensions escalated when Williams arrived at the Mercer Island home after Cuvillier canceled a planned visit. When Cuvillier refused to allow Williams inside, he called 911. The confrontation turned physical, and Cuvillier was subsequently charged with fourth-degree assault in Mercer Island Municipal Court.
Shortly after the January incident, Cuvillier filed a petition for a protection order against Williams on behalf of Dominick. In the petition, she stated that Williams’ behavior had been unpredictable and aggressive. She expressed particular concern about the number of firearms Williams kept in his home.
In a subsequent document, Williams acknowledged possessing 53 firearms, which he surrendered to authorities. The protection order was granted and reissued three additional times throughout the year.
In August, Cuvillier and Williams reached an agreement regarding Dominick’s care. Under the arrangement, Dominick would live with his mother, while Williams would have visitation rights. The agreement specified that Cuvillier and Williams would communicate only through text messages or email to shield Dominick from their conflicts. Cuvillier was named as Dominick’s sole guardian and conservator.
The protection order was terminated in early December, weeks before the deaths occurred on December 31, 2025.
Neighbors on Mercer Island expressed shock at the deaths. One resident said he regularly brought his dog to play with Cuvillier’s two dogs and had walked past her residence before police arrived Tuesday morning.
In Issaquah, a neighbor who lived next door to Williams and his wife described them as considerate neighbors. Williams’ wife maintained a garden, and the couple was known for being meticulous about their property.
Mercer Island, located on Lake Washington near Seattle, ranks among the wealthiest communities in the United States. The community experiences one of the lowest homicide rates in Washington State, with murder cases typically occurring several years apart.
Issaquah is a suburban community in King County near Seattle.
Cuvillier was known in the community as a dedicated artist with a focus on photography, regularly sharing her work on social media platforms. Local businesses displayed her pieces.
Authorities from both Mercer Island and Issaquah police departments continue working to establish the sequence of events and determine which deaths occurred first. Investigators have not determined the connection between the timing of the deaths at the two separate locations.
The King County Medical Examiner’s office will conduct autopsies to determine the official cause and manner of death for all four individuals. All firearms recovered from both scenes will undergo forensic analysis as part of the ongoing investigation.