The four people who died in a suspected homicide-suicide in Mercer Island and Issaquah homes were related, Mercer Island and Issaquah police said.
Danielle Cuvillier, 80, and her son, Mackenzie P. Williams, 45, were found dead Tuesday morning at a Mercer Island home in the 8400 block of Southeast 46th Street. Later in the day, a second adult son and a woman in her 40s were found dead in an Issaquah house in the 400 block of Southeast Evans Lane, where police were doing a welfare check.
The Mercer Island Police Department identified Cuvillier and Williams on Wednesday afternoon. The other two have not been publicly identified.
The Mercer Island deaths are being investigated as a homicide and suicide, police Cmdr. Jeff Magnan said. It is unclear who shot the other before turning the gun on themself.
After Mercer Island police responded to the deaths of Cuvillier and Williams, they asked Issaquah police to do a welfare check on the woman's other son, who was in his 30s, in Issaquah, Magnan said Wednesday. The son was found dead along with a woman in her 40s. The woman and one of the sons were married, according to Issaquah police.
The two people in Issaquah had been shot, and there were no signs of forced entry.
Four firearms were found inside the Mercer Island home and a fifth firearm was found in a car. The two addresses are about 14 miles apart.
The King County medical examiner’s office will determine the cause and manner of the deaths.
Neighbors ‘hurt for all of them’
Mercer Island police officers were at the 2,840-square-foot house that Cuvillier had owned since 1995 on Wednesday.
“It’s pretty startling,” said Don Strome, a neighbor.
Strome used to bring his dog, Minny, over to play with Cuvillier’s two dogs. He walked Minny by her home about an hour before police arrived.
“She always had a smile on her face and was wonderful at taking care of her son and taking care of those two dogs,” Strome said.
In Issaquah, police officers processing the crime scene carried rifles out of the home with gloved hands.
Neighbors behind the home said they heard what may have been gunfire inside the home around 8 p.m. Monday. The next day, before noon, they said their streets were blocked off by police.
Shelby Rosales has lived next door to Williams and his wife for five years. To her, “they were very considerate and thoughtful neighbors.”
Williams’ wife grew cosmos, tomatillos, sunflowers and squash, Rosales said. They talked about gardening together and even did a bulb swap.
When a bear dragged trash from the older son and his wife’s yard, Rosales said Williams came over to pick up all the trash down to the small grains of rice strewn between the grass.
“It’s been all consuming,” thinking over their deaths, Rosales said. “It hurts. It’s sad. I hurt for all of them.”
Months of conflict
The deaths were preceded by more than a year of conflict between Cuvillier and Williams about the younger son’s care, according to court documents.
The 34-year-old had Angelman syndrome, a rare genetic disorder that causes severe developmental delays, and needed 24-hour care. He lived with his mother and father until his father’s death in 2018, when Cuvillier and Williams began a shared, informal caregiving arrangement for him.
Over the last few years, Williams had become increasingly concerned about his brother’s care under his mother, stating in a King County court declaration that he witnessed her being physically and emotionally abusive. Cuvillier denied the allegations.
Williams met with a lawyer, and in January 2025, worked with his younger brother to create a new power of attorney so the older brother would be responsible for his sibling’s affairs.
In response, Cuvillier said her younger son had been “unduly influenced” to execute a new power of attorney. Williams came to the Mercer Island house after Cuvillier canceled a planned visit, and he called 911 when she wouldn’t let him in. The two fought, and Cuvillier was charged with fourth-degree assault in Mercer Island Municipal Court.
Cuvillier soon after filed a petition for a protection order against her older son on behalf of her younger son, saying Williams’ “behavior has been unpredictable and aggressive lately.” She wrote she was concerned about the number of firearms at his house — he wrote he had more than 50 in a subsequent document where he surrendered the firearms. The protection order was granted and reissued three more times.
The mother and son came to a resolution in August where the younger son would live with the mother, and Cuvillier was named as his guardian and conservator. The two agreed the younger son would visit his brother on Sundays and Mondays and spend one weekend per month with him. They agreed they would only communicate through text message or email, and wouldn’t disparage the other to the younger son.
The protection order was terminated in early December. Less than three weeks later, all three were dead.
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