An autopsy was completed on Tuesday afternoon but the official cause of death has not been released. CBC News has learned that the child died as a result of drowning.
The boy, a Grade 2 student, lived with his parents and older sister on the main floor at the Chateau Apartments in the Pleasantview neighbourhood at 109th Street and 53rd Avenue.
Paramedics rushed the child to hospital after he was found inside the apartment around 8:30 a.m. but he was pronounced dead shortly after arrival.
Police later took the boy's 32-year-old mother away for questioning. Only the mother and the boy were home at the time.
The woman remains in custody but no charges have been laid.
A neighbour was devastated when told about the boy's death.
Amher Geling told CBC News she last saw the boy Monday night around 7:30 p.m. when she let him and his sister into the building as they did not have a key.
She called the victim a "normal boy," who was always with his sister. The family had moved into the building last year, she said.
Ken Brompton lives in the building and says he awoke to the sounds of sirens. He said police knocked on his door and wanted to know if he had heard anything.
"They were interviewing everyone in the building, I think," he said.
The boy's death is Edmonton's third homicide of 2013.