MOSCOW — A man sentenced to death in connection with the bombing of a subway station in Belarus has been executed, the man’s family said Saturday.

The man, Vladislav Kovalyov, 26, was convicted of helping his childhood friend carry out the April 2011 bombing, but human rights groups and Western governments have cast serious doubt on the verdict, pointing to flaws in the investigation.

Confirmation of Mr. Kovalyov’s execution came in a letter from the Supreme Court delivered on Saturday to his family’s residence in Vitsyebsk, a town in northeastern Belarus. Tatyana Kozyar, Mr. Kovalyov’s sister, uploaded a copy of the letter to the Russian social networking site VKontakte with the caption “They’ve killed Vlad.”

The letter, dated March 16, said, “I inform you that the sentence of the Supreme Court of the Republic of Belarus from 30 November 2011 in relation to your son, Kovalyov Vladislav Yuryevich, has been carried out.”