The assassination or Mr. Nemtsov, a charismatic opposition leader who antagonized the Kremlin, was the most prominent political murder in Russia since President Vladimir V. Putin came to power in 1999.
Investigators continue to collect evidence concerning who ordered the killing, according to Svetlana Petrenko, a spokeswoman for Russia’s Investigative Committee. But Mr. Nemtsov’s supporters are skeptical that the authorities will bring the organizers to justice. Evidence in the case points to high-ranking government officials in Russia’s restive republic of Chechnya, led by Mr. Kadyrov, an ally of Mr. Putin who enjoys the support and protection of the Kremlin.
Prosecutors have said that Mr. Dadayev and his four accomplices killed Mr. Nemtsov after Ruslan Mukhudinov, the driver of a high-ranking Chechen military officer, promised to pay them $250,000. The prosecutors failed to adequately explain why the driver would have that money or what motivated him to organize the killing.
Mr. Mukhudinov was charged in absentia, after he fled to the United Arab Emirates. His boss, Ruslan Geremeyev, deputy head of one of the main military regiments in Chechnya and a confidant of Mr. Kadyrov, was never questioned in the case. Mr. Geremeyev did not show up for questioning and, when investigators visited his home in Chechnya, no one answered the door, officials said.
Some observers have said that Mr. Kadyrov wanted to test the limits of his independence from the Kremlin. Others have said he wanted to help his patron, Mr. Putin, by eliminating one of his harshest critics, even though the killing embarrassed the Kremlin.
The Kremlin’s spokesman, Dmitri S. Peskov, told journalists on Thursday that “there is hope” that the organizers would still be found. Shortly after the killing, Mr. Putin called it “cruel” and “provocative,” and he promised Mr. Nemtsov’s mother that he would “do everything” to see that the organizers of her son’s murder would be punished.
As he quickly rose through the ranks of the Russian government in the 1990s after the fall of the Soviet Union, Mr. Nemtsov turned into an advocate of democratic reforms. After Mr. Putin became president, Mr. Nemtsov’s rise stalled and he turned into a sharp critic of the government.