Laura Shatto told deputies that she was inside the family's home outside of Odessa, Texas, on Jan. 21 and came out to find Max Alan Shatto on the ground, Ector County Sheriff Mark Donaldson said.
Deputies arrived at the home as the ambulance was leaving with Max, he said, and the boy was pronounced dead at the hospital a short time later. No one has been arrested in the case, authorities said.
The mother said Max was playing with his biological brother, 2-year-old Kristopher Shatto. Donaldson said he wasn't sure if anyone else was at the home besides Laura Shatto and the boys, whom Shatto and her husband Alan recently adopted.
The medical examiner's office said it couldn't immediately be determined if bruises on several parts of Max's body were intentional or accidental, but Russian authorities have blamed Max's death on "inhuman treatment" by the adopted parents.
Shirley Standefer, chief investigator for the Ector County Medical Examiner's Office, says there were signs of bruising on Max's lower abdominal area.
A full autopsy will be needed to determine what kind of bruising was on Max's body, she said. Authorities also have not received a toxicology report that would have details on whether Max was being given any medication, she said.
Donaldson said he didn't know when the final autopsy report would be completed.
A preliminary report from the Tarrant County Medical Examiner's Office has been received by county officials, but they decline to release it.
The death comes weeks after Russia announced it was banning all American adoptions in retaliation for a new U.S. law targeting alleged Russian human-rights violators. The ban also reflects lingering resentment over the 60,000 Russian children adopted by Americans in the past two decades, of which at least 19 have died.
Texas Child Protective Services spokesman Patrick Crimmins confirmed the agency had received a report on Max's death Jan. 21. Crimmins said CPS had received allegations of physical abuse and neglect, but had not determined whether those allegations were true.
The other boy, Kristopher Shatto, remains with his adoptive parents, Crimmins said.