Pavel Astakhov wrote on his Twitter feed that Texas Child Protective Services is looking into the death Jan. 21 of Maxim Shatto, 3. The boy had been badly beaten and given powerful "psychotropic substances," the Russia news agency RIA Novosti reported.
Astakhov told a television interviewer Maxim's adoptive mother Laura Shatto was currently restricted to seeing her other child just once a week.
A representative for Texas Child Protective Services confirmed that the agency was investigating the boy's death along with officials from the Ector County Sheriff's Department in Odessa, Texas.
"In this particular case, the allegations reported to CPS were physical abuse and neglectful supervision, or simple, neglect," the representative said.
Over the past two decades more than 60,000 Russian children have been adopted by Americans.
Late last year, Russia's parliament banned adoptions by U.S. nationals following passage of the Magnitsky Act, which placed financial and travel restrictions on Russian officials for violating human rights.