Police said they found four firearms inside the home, which had ‘no signs of forced entry’.
During the investigation, officers ascertained that someone was missing from the property and carried out a check on another home, a short drive away in Issaquah, which reportedly belonged to Williams.
There, they found the bodies of a woman in her 40s and a man in his 30s, believed to be another son and a daughter-in-law.
The Mercer Island Police Department (MIPD) said on Tuesday: “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.”
They added: “Investigators continue to work closely with the King County Medical Examiner’s Office to determine the official cause and manner of death.”
Court records relating to both addresses show that Williams had been engaged in a series of legal disputes with his mother, Cuvillier, largely over the care of his disabled brother.
These documents also showed that Williams was forced to surrender over 50 guns to local police after concerns, according to King 5 Seattle.
The station obtained a 911 call recording that appears to show the pair arguing about the younger son’s care. After that call, Cuvillier was arrested and spent a night in jail.
Williams accused his mother of being physically and emotionally abusive in court filings, The Seattle Times has reported. But after a year-long struggle over custody, Cuvillier was granted guardianship over her younger son.
As part of the court ruling, a visitation agreement was drawn up where Williams could see his brother. Court documents show that the visitation schedule required him to return his brother to the Mercer Island property by Monday night, the day before the bodies were found.
In an update to the public on New Year’s Eve, the MIPD said: “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.”
Officers are yet to confirm who was responsible for the shootings while they await the outcomes of the four autopsies, as well as ballistic analysis from the two scenes.