Three-year-old Max Shatto, also known as Maxim Kuzmin, died on Jan. 21 in Ector County, Texas, RIA Novosti reported.
On Friday, Ector County District Attorney Bobby Bland cited medical examiners on Friday when declaring the boy's death had been accidental.
The announcement came after Russia's child rights ombudsman, Pavel Astakhov, last week accused the boy's adoptive mother of killing Shatto and feeding him "psychotropic substances."
Medical examiners in Texas said they found no drugs in the boy's system and bruises that were on his body were consistent with self-inflicted injuries.
"His bruises disappeared, the medicine vanished, his adoptive parents were acquitted and the authorities have backtracked," Astakhov tweeted Saturday. "The 3-year-old boy became a victim of big politics."
Irina Yarovaya, the head of the Russian parliament's security committee, said the U.S. investigation of the boy's death were suspicious.
"The obvious contradictions and the mystery around the goings-on give serious cause not to trust this announcement," Yarovaya said.
The Russian Foreign Ministry's commissioner for human rights, Konstantin Dolgov, Russian has requested all the case materials related to Shatto's death.
Shatto and hit 2-year-old brother were adopted by a Texas couple in November -- shortly before Russia banned all U.S. adoptions of Russian children.
Russia's ban was in retaliation of the United State's Magnitsky Act, which imposed financial and travel restrictions on Russian officials deemed by the United States to have violated human rights.