MOSCOW — Five men suspected of involvement in the killing of opposition leader Boris Nemtsov pleaded not guilty on Monday.

Nemtsov was shot late at night in February 2015 as he and a companion walked across a bridge near the Kremlin. The brutality so close to the center of Russian power both frightened and angered supporters of the beleaguered opposition.

Five suspects, all of them Chechens, faced the court in Moscow on Monday as the hearings began. The suspected triggerman who served as an officer in the security forces of the Moscow-backed Chechen leader, Ramzan Kadyrov, as well as four other people pleaded not guilty to the charges.

The official probe has failed to identify those who ordered the killing, and Russian opposition activists have criticized the Kremlin for failing to track down the mastermind.

“Nemtsov’s murder hasn’t been solved,” said Vadim Prokhorov who represents Nemtsov’s family in the trial.

“We hope that this court hearing will help to get additional arguments to force investigators and the government to conduct a proper investigation not only those into who carried it out but against those who have plotted this murder.”

A separate investigation has declared the driver of an influential Chechen commander as the likeliest mastermind, a suggestion that Nemtsov’s family find laughable.