(Reuters) - A colleague of Boris Nemtsov, the Russian opposition figure shot dead near Moscow's Red Square, said suggestions he was killed by Islamists were nonsensical and useful for the Kremlin because they deflected accusations that officials were involved.

Speculation about an Islamist link strengthened after investigators charged a man from the Muslim Chechnya region over the killing, and his former boss said the suspect had been angered by publication of caricatures of the Prophet Mohammad in French satirical magazine Charlie Hebdo.

That has been met with scepticism by some of Nemtsov's associates. They believe that the Kremlin stood to gain from the killing -- though Russian officials have denied involvement -- and they do not believe fanatics acting alone could have shot someone dead so close to the Kremlin.

"Our worst fears are coming true," Ilya Yashin, the co-leader of Nemtsov's small liberal opposition party said on Twitter late on Sunday. "The trigger man will be blamed, while those who actually ordered Nemtsov's killing will go free."

"Investigators' nonsensical theory about Islamist motives in Nemtsov's killing suit the Kremlin and take Putin out of the firing line," Yashin added on Monday, referring to Russian President Vladimir Putin.

Nemtsov, a 55-year-old former deputy prime minister who became a vocal Kremlin critic, was shot in the back four times as he walked home near Red Square on Feb. 27. His was the most high-profile killing of an opposition figure in Putin's 15 years in power.

Nemtsov was not widely popular in Russia outside the small, urban intelligentsia. But his supporters say he was a threat to the Kremlin because he was determined to expose official corruption.

In the days before he was killed, he was working on a report which, aides said, would allege that Russia was sending regular troops to fight in eastern Ukraine. Moscow has denied any direct involvement in the fighting.

Prosecutors have charged two men, Zaur Dadayev and Anzor Gubashev, with involvement in Nemtsov's killing, and are holding three others as suspects. Officials say Dadayev, who used to be a commander in the Chechen police's "Sever" battalion, has admitted involvement.

Chechen leader Ramzan Kadyrov, who is loyal to Putin, said he knew Dadayev and described him as a true patriot.

"All who know Zaur (Dadayev) confirm that he is a deep believer and also that he, like all Muslims, was shocked by the activities of Charlie and comments in support of printing the cartoons," Kadyrov wrote on his Instagram account.

Nemtsov had defended the cartoons of the Prophet Mohammed published in the French satirical magazine Charlie Hebdo.

Putin has called Nemtsov's killing as a shameful tragedy. Officials have said they will conduct a thorough investigation. They say they are still looking for people they believe were involved in the killing.