MOSCOW — Russian lawmakers on Friday demanded the return of a 2-year-old boy whose 3-year-old brother, adopted by the same family in Texas, died under murky circumstances in January, prompting allegations of abuse and an investigation.

The younger boy, Kristopher Shatto, who was born Kirill Kuzmin in Russia, remains in the custody of his adoptive parents, Alan and Laura Shatto, in Gardendale, Tex. His biological mother, Yulia V. Kuzmina, who lost custody of the boys because of alcohol addiction, has also demanded Kirill’s return.

But even as the State Duma, the lower house of Parliament, adopted a resolution calling for Kirill to be brought back to Russia, news outlets here reported that Ms. Kuzmina and her boyfriend had gotten into a drunken brawl and were removed from a train by the police while returning to Gdov, a town near the border with Estonia. They had been in Moscow for a television appearance.

Yevgeny Usovich, the owner of a resort on Lake Peipus where Ms. Kuzmina’s boyfriend, Vladimir Antipenko, worked as a bathhouse attendant, said he had seen the couple after their return but was unable to get details of their travel. “It was useless; they were drunk,” Mr. Usovich said in a telephone interview. “What could I find from them?”