MOSCOW — Russian authorities appeared Monday to bolster claims that opposition activist Boris Nemtsov was killed by an Islamist gunman over perceived criticism of Muslims, deepening skepticism among Nemtsov’s supporters that justice will be served.

The trajectory of the Russian probe into the Feb. 27 slaying shifts attention away from potential motives related to Nemtsov’s strident opposition of President Vladimir Putin, which the Kremlin critic’s allies insist was the reason he was targeted.

“Investigators’ nonsensical theory about Islamist motives in Nemtsov’s killing suit the Kremlin and take Putin out of the firing line,” Ilya Yashin, a close political ally of Nemtsov, tweeted Monday.

Russian news outlets, citing unnamed sources, identified the shooter as Zaur Dadayev, a suspect who reportedly confessed Sunday to a role in the slaying.

According to a report on the Russian news site Rosbalt, Dadayev was upset by what he viewed as Nemtsov’s anti-Islam stance and criticism of Muslims, including denunciations of a deadly attack in January on the Paris satirical publication Charlie Hebdo. The weekly had angered many Muslims by printing cartoons depicting the prophet Muhammad. Two brothers of Algerian descent killed 12 people in the attack on the publication’s office.

The Rosbalt report said Dadayev enlisted relatives in Moscow to help carry out the Nemtsov hit, which occurred on a bridge within sight of the Kremlin. Five men were arrested Sunday in the case.

Russian news reports said that the suspects were ethnic Chechens and that Dadayev had previously served as a police officer in Chechnya, where Muslim separatists have waged battles against Russian forces for decades.

Chechen leader Ramzan Kadyrov commented on his Instagram account Sunday that Dadayev was a “true patriot of Russia” and a deeply devout man who, “like all Muslims, was very shocked by the actions of Charlie (Hebdo) and by comments supporting the printing of the caricatures.”

On Monday, Putin gave Kadyrov a state award, the Order of Honor, which is bestowed for achievements in public service.

Kadyrov, a Moscow ally who has led the Chechen Republic since 2007, has been widely criticized for a crackdown on dissent. But Putin has lauded him for his support in battling Islamist separatists in the region.

Chechens and other residents of the North Caucasus have been charged in previous killings of Kremlin critics, such as the 2006 murder of journalist Anna Politkovskaya. The conviction of five men in her case last year did not satisfy her supporters, however. They suspect that those who ordered her killing are still free.

The arrest of the five ethnic Chechens in Nemtsov’s case has erased the opposition’s initial, cautious hopes that the real reason for the killing would be made known.

“Our worst fears are coming true,” Yashin, the Moscow chairman of the Nemtsov-led RPR-PARNAS party, wrote on Twitter on Sunday. “The trigger man will be blamed, while those who actually ordered Nemtsov’s killing will go free.”

The opposition says the Kremlin played at least a passive role in Nemtsov’s killing, which occurred in one of the most heavily policed areas of the country. It took emergency vehicles 11 minutes to arrive at the scene after assailants shot him four times in the back and disappeared in a car, according to surveillance footage broadcast in Moscow.

Members of the opposition say they have felt targeted ever since Putin has stepped up rhetoric about “national traitors” and a “fifth column” trying to undermine Russia from within. Even Nemtsov said last month that his relatives feared for his life.