MOSCOW — A Russian jury convicted five men on Thursday in the 2015 assassination of Boris Y. Nemtsov, the opposition leader and former first deputy prime minister, but his family called the trial a cover-up and said that those who had ordered and arranged for the killing had not been brought to justice.

Mr. Nemtsov, 55, was gunned down on the evening of Feb. 27, 2015, as he walked across a bridge outside the Kremlin’s crimson walls after dining with his girlfriend in a restaurant on Red Square.

The shooting, days before Mr. Nemtsov was to lead a rally to protest the war in Ukraine, ended his two-decade career as a champion of democratic reforms, beginning after the Soviet Union’s collapse in 1991.

The assassination, the most prominent political killing since Vladimir V. Putin took power in 1999, brought thousands of mourners into the streets but also frightened the country’s embattled democratic opposition. Mr. Putin called the killing “brazen” and politically motivated, and the Kremlin has denied any involvement.