It was the first time that police officially ruled out another killer in the July deaths that shocked a quiet suburban area of the Manitoba capital.
"This is in fact a homicide-suicide incident ... and really, I can't get into any further details than that," Const. Jason Michalyshen said.
Michalyshen acknowledged there are many unanswered questions about what happened to two-year-old Anna and her infant brother Nicholas. Police have remained tight-lipped on the case.
So far, they have only said that someone — they won't say who — called police on the morning of the killings and asked them to check on the kids. Gibson disappeared at some point. Her body was found three days later in the Red River.
Michalyshen confirmed the children's deaths were "consistent" with drowning, but would not go any further. The head of city's firefighters' union had earlier said the kids were found unresponsive in the bathtub.
It remains unclear how long it took first responders to find the children in the tub, and whether a relative found them first. Michalyshen said an internal "assessment" is underway, but its findings would not likely be made public.
"Senior members are in the process of putting this assessment together and trying to determine how things played out."
Media reports have said Gibson suffered from postpartum depression, and there have been calls for an inquest to examine what kind of health care she may have received following the birth of her son in the spring.
The office of Manitoba's chief medical examiner has said it will be late fall before there's a decision on whether to call a provincial inquest, which would include sworn testimony from medical experts and others in open court.
Hundreds of people attended a memorial in August for Gibson, who was 32 at the time of her death, and her two children.