An email sent by the son three hours before the bodies were found in Issaquah, Mercer Island gave clues that his life was about to end.
MERCER ISLAND, Wash. — Police suspect 45-year-old Mackenzie “Mack” Williams was the shooter in all three of his family members’ deaths, before turning the gun on himself, multiple sources with direct knowledge of the events confirm to KING 5.
KING 5 has exclusive information that Williams, an unemployed computer professional, sent an email to friends just hours before the bodies were found — a message that appeared to foreshadow what was about to happen.
Three hours before police made their discoveries, Williams wrote an email focused on his gun collection, according to sources. In it, he acknowledged that anyone attempting to retrieve his weapons might encounter something traumatic. “If any or all of you want to make some effort to recover my guns and associated property then you are welcome to it,” Williams wrote. “It may require you to go into my house and see some things that you would prefer not to. Just walk away if you don’t want to deal with it.”
The email alarmed those who received it and prompted a call to 911 requesting a welfare check.
Hours later, four members of the family were found dead in homes on Mercer Island and Issaquah on Dec. 30, 2025. First, around 10:45 a.m., police discovered the bodies of Danielle Cuvillier, and her son, Williams, inside the mother’s home on Mercer Island. Both died from gunshot wounds and preliminary information led police to believe the incident was a homicide, then a suicide.
Shortly thereafter, officers in Issaquah conducted a welfare check at Williams’ Issaquah home on the 400 block of Southeast Evans Lane. KING 5 has confirmed the people found in the Issaquah home are Harmony Danner, Williams’ wife, and Dominick “Nick” Cuvillier, Williams’ disabled younger brother.
Multiple sources confirm police believe Williams shot and killed his wife and his brother in Issaquah, then killed his mother in her home on Mercer Island, before shooting and killing himself.
Williams’ Dec. 30 email went on to say, “My interest is concluded. Sorry to part on these terms. Good luck.”
The memo did not reveal any insight into a motive.
Family history shows custody battle
Court records reveal a year-long battle over the care and guardianship of Nick, Williams’ brother who has Angelman syndrome, a rare genetic disorder that left him mostly non-verbal and reliant on assistance for activities of daily living.
A year ago, Danielle Cuvillier filed paperwork for a protection order against her son Williams. She cited concerns about his mental stability, unpredictable behavior and access to firearms, KING 5 previously reported. After the order was filed, court documents show Mack Williams turned over 53 firearms to the Issaquah Police Department. A bio for Williams states he graduated from the University of Washington with an engineering degree in 2006. Friends tell KING he’d been unemployed for several months.
The protection order against Williams was dropped approximately three weeks ago.
Court records also show that Williams said his mother “stated several times that she intended to kill Nick and then herself,” prompting Williams and his father to remove the father’s firearms and secure them in Williams’ gun safe seven years ago. Williams said the statements and his mother’s behavior caused him increasing concern for Nick’s safety.
A 911 call in January 2025 recorded a verbal and physical confrontation between Williams and his mother, Danielle; she was charged with fourth-degree assault in Mercer Island Municipal Court. The fight was about Mack being able to visit his brother. Records show Mack and his wife Harmony regularly cared for Nick.
The incident, Williams said in court records, led him to contacting Adult Protective Services and request custody of Nick, who stayed with Williams and his wife for about two weeks afterward.
If you or someone you know is in crisis, help is available. Call or text 988 or chat at 988lifeline.org. Visit Vibrant Emotional Health’s Safe Space for digital resources.
KING 5’s Dalton Day contributed information to this story.