MOSCOW (Reuters) - Russia criticized on Tuesday a decision by U.S. authorities not to press charges against the adoptive parents of a Russian-born boy who died in Texas in January, saying it "raised serious questions".

The death of three-year-old Max Shatto led to criminal and child welfare investigations in the U.S. state, Russia opened its own inquiry and lawmakers in Moscow called for his younger brother to be returned to Russia.

The dispute, following a Russian ban on U.S. adoptions of its children, has added to strains between Washington and Moscow since Vladimir Putin returned to the presidency last year.

A U.S. district attorney said on Monday that the couple who adopted the boy and his younger brother would not face criminal charges. He described the death as an accident.

Shatto, who is referred to in Russia by his Russian name of Maksim Kuzmin, was unresponsive when his adoptive mother found him in the yard of their home in Gardendale, Texas. He died at a nearby hospital.

"The decision by the authorities of the state of Texas not to press charges against the Shattos in regard to the tragic death of Maksim Kuzmin raises serious questions," Russia's Foreign Ministry said in a statement.

"We count on gaining unimpeded consular access to the younger brother of Maksim Kuzmin, Kirill, in order to determine the real conditions in which he is living in the United States."

Moscow has seized on the death of the boy to justify the adoption ban, which was imposed at the start of this year.

The Russian ban had been introduced after the U.S. Congress passed legislation aimed at punishing Russians accused of violating human rights.

(Reporting by Thomas Grove, Editing by Timothy Heritage)