Supporters and allies of the slain Boris Y. Nemtsov have expressed fear that the opposition leader's killing may have come at the behest of the government, the Washington Post reports. The government, meanwhile, has suggested that Nemtsov may have been martyred by other members of the opposition.

"There is only one conclusion," Nemtsovally Vladimir Milov, who the Post describes as an opposition leader and ally of Nemtsov's, reportedly wrote on his blog. "The murder of Boris Nemtsov is connected to the authorities."

"The personal perception of safety has just been enormously shattered. No one considered that someone could be just shot down. The regime was used to imprisoning people," said Leonid Volkov, with whom Nemtsov had been organizing a rally originally scheduled for Sunday. "It's a new era in Russian opposition politics."

"It's not decided, but it could go both directions. Toward more cruelty or actually some change in the regime, as well, if we figure out how to use this momentum," Volkov said.

Meanwhile, the Investigative Committee of the Prosecutor General's Office released a statement enumerating several theories about the killing, including the possibility that Nemtsov—a former deputy prime minister—was martyred by other members of the opposition, the New York Times reports.

"The investigation is considering several versions," the committee said, suggesting that the murder might have been "a provocation to destabilize the political situation in the country, where the figure of Nemtsov could have become a sort of sacrificial victim for those who stop at nothing to achieve their political goals."

"The president noted that this cruel murder has all the signs of a contract killing and carries an exclusively provocative character," the Krelmin said in a statement. "Vladimir Putin expressed his deep condolences to the relatives and loved ones of Boris Nemtsov, who died tragically." According to the Post, a Kremlin spokesman said that there was no reason to believe that other opposition leaders would be killed.

According to the Times, the investigative committee's statement also referred to the possibility that Islamic extremists were responsible for the killing, citing Nemtsov's position on the Charlie Hebdo shootings. On Saturday, state-run news outlets reported that investigators had found a car that they believe was used in the shooting with license plates from Ingushetia, a Muslim-majority province in the Caucasus, the Post reports.

The Times also reports that about two weeks before his death, Nemtsov met with Yevgenia Albats, the editor of New Times magazine, to discuss research he was conducting on Russia's involvement in Ukraine.

"He was afraid of being killed," Albats told the Times. "And he was trying to convince himself, and me, they wouldn't touch him because he was a member of the Russian government, a vice premier, and they wouldn't want to create a precedent. Because as he said, one time the power will change hands in Russia again, and those who served Putin wouldn't want to create this precedent."