Mercer Island Police Investigate Deaths Following Welfare Check

January 3, 2026 Update

On January 2, 2026, the Mercer Island Police Department received confirmation from the King County Medical Examiner’s Office regarding the cause and manner of death in the December 30, 2025, incident. The preliminary results are as follows:

The Medical Examiner ruled the death of Mackenzie P. Williams a suicide and the death of Danielle Cuvillier a homicide.

The Mercer Island Police Department’s investigation remains ongoing. Detectives continue to review evidence, including forensic analysis of firearms recovered from the scene. Additional information will be released as it becomes available.

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December 31, 2025 Update

The Mercer Island Police Department has confirmed the identities of the two individuals found deceased on December 30, 2025, at a residence in the 8400 block of SE 46th Street.

Positive identification was made through Washington State driver’s licenses. The female was identified as Danielle Cuvillier, born in 1945. The male was identified as her son, Mackenzie P. Williams, born in 1980.

Investigators continue to work closely with the King County Medical Examiner’s Office to determine the official cause and manner of death.

A total of four firearms were recovered from within the residence, along with one additional firearm located in a vehicle that was impounded as part of the investigation.

All firearms have been secured and will undergo forensic analysis. This process will take time, and results will be released when available.

The investigation remains ongoing. Additional information will be provided as it becomes appropriate to do so.

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Mercer Island, WA — December 30, 2025

At approximately 10:45 a.m. on December 30, 2025, Mercer Island Police Department (MIPD) officers were dispatched to a residence in the 8400 block of SE 46th Street to conduct a welfare check.

The request originated from an attorney representing the homeowner, who reported receiving a concerning email that raised fears for the occupants’ safety.

Upon arrival, officers looked through a window and observed an individual inside the residence who appeared to have suffered a gunshot wound. Officers made entry into the home and located two deceased individuals: a female in her late 70s and a male in his late 40s. Firearms were found within the residence and in proximity to the individuals.

Based on preliminary information, the incident appears to be a homicide followed by a suicide. There is no indication of an ongoing threat to the community. Investigators did identify concern for another individual who had previously resided at the location. As a precaution, MIPD requested the Issaquah Police Department conduct a welfare check at a separate residence in their city.

Detectives and members of the Washington State Patrol Crime Scene Response Team are actively investigating the circumstances surrounding the incident. This includes neighborhood canvassing and coordination with the King County Medical Examiner’s Office, which will determine the official cause and manner of death.

Further information will be released as it becomes available.

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For media inquiries, please contact:

Commander Jeff Magnan

Public Information Officer

Mercer Island Police Department

jeff.magnan@mercerisland.gov