All point to Putin, but other suspects can't quite be ruled out. Andrew E. Kramer, writing in the NYT, has some background.
MOSCOW — About two weeks before he was shot and killed in the highest-profile political assassination in Russia in a decade, Boris Y. Nemtsov met with an old friend to discuss his latest research into what he said was dissembling and misdeeds in the Kremlin.
He was, as always, pugilistic and excited, saying he wanted to publish the research in a pamphlet to be called “Putin and the War,” about President Vladimir V. Putin and Russian involvement in the Ukraine conflict, recalled Yevgenia Albats, the editor of New Times magazine. Both knew the stakes.
Mr. Nemtsov, a former deputy prime minister, knew his work was dangerous but tried to convince her that, as a former high official in the Kremlin, he enjoyed immunity, Ms. Albats said.
“He was afraid of being killed,” Ms. Albats said. “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.”
Putin or his spokespople had an imaginative list of suspects:
Russian authorities said on Saturday that one line of investigation would be to examine whether Mr. Nemtsov, a 55-year-old former first deputy prime minister and longtime leader of the opposition, had become a “sacrificial victim” to rally support for opponents of the government, the Investigative Committee of the Prosecutor General’s Office said in a statement.
The economic crisis provoked by the collapse of oil prices, with a minor assist from Western sanctions, is already causing some economic pain, not least for the crony capitalists that form a hard core of Putin support.
This comes as analysts of Russian politics say the Kremlin could be worried about, and intent on discouraging, further defections to the opposition, given reported high-level schisms between hard-liners and liberals over military and economic policy. The government is already under strain from Russia’s unacknowledged involvement in the war in Ukraine and runaway inflation in an economic crisis.
A high profile murder might be considered a good way to discourage "further defections."
About that research and pamphlet. Will we now ever see it?