The parents of the child asked shaman So Dyavor, 59, and her husband, Kim Sende, 62, to perform a ritual to exorcise “evil spirits” that they believed were plaguing him, the local news web site Primamedia.ru reported.
The child stopped breathing during the ritual in the local village of Sergeyevka on Saturday.
No traces of violence were found on the boy's body, and forensic pathologists on Wednesday had not established what killed him.
It remains possible that the boy's pneumonia was the cause of his death, a police spokesman told RIA-Novosti.
The tabloid Tvoi Den identified the boy as Dmitry Kazachuk and said he arrived in Sergeyevka with a delegation of relatives that included his mother, aunt, uncle and grandmother.
The family intended to request help for the grandmother, who has diabetes, but So Dyavor told them that the entire family was jinxed and the boy had put a curse on them, the report said.
Nobody was present in the room when the shaman performed the exorcism on the boy, it said, without commenting on the role of So Dyavor's husband in the incident.
The local branch of the Investigative Committee has opened a criminal case into suspected negligent homicide, which is punishable by up to three years in prison, but has not charged anyone, Interfax said.