Ombudsman Pavel Astakhov tweeted that U.S. prosecutors failed to scrutinize the circumstances of 3-year-old Maxim Kuzmin's death, the Russian news agency RIA Novosti reported.
Kuzmin, who is known in the United States by his adoptive name of Max Shatto, died on January 21 nearly three months after he and his younger brother were adopted from a Russian orphanage.
Medical officials in Texas said Max had a history of "self-injury."
The Ector County district attorney told RIA Novosti that Texas Child Protective Services is still investigating the toddler's death.
Russia's Investigative Committee is also looking into the death.