It was the most prominent political murder in Russia since President Vladimir V. Putin came to power.
Shortly before midnight on Feb. 27, 2015, the opposition leader Boris Y. Nemtsov was gunned down a few steps away from the Kremlin as he and his girlfriend were walking home across a bridge from Red Square.
On Thursday, a court in Moscow sentenced the Chechen former security services officer, who was convicted of pulling the trigger, to 20 years in prison; four accomplices were ordered to serve 11 to 19 years.
The shooting of Mr. Nemtsov, a charismatic opposition leader who antagonized the Kremlin, ended his two-decade democratic crusade that began in the wake of the Soviet Union’s collapse in 1991, and continued through the authoritarian rise of Mr. Putin.
An atomic physicist by training, Mr. Nemtsov became active in politics in the late 1980s and organized protests against the planned construction of nuclear reactors in his home city, Nizhny Novgorod, about 250 miles east of Moscow.
After the end of Communist rule, he became a vocal proponent of pluralistic democracy under President Boris N. Yeltsin, who appointed him governor of Nizhny Novgorod in 1992. Mr. Nemtsov was just 32 years old at the time, a handsome and confident member of a new breed of Russian leaders who, with an eye on the West, foresaw an ambitious path forward for their country and its ailing economy.
Mr. Nemtsov went on to become a deputy prime minister in Mr. Yeltsin’s administration, and he had been regarded as the president’s heir apparent. But Mr. Nemtsov, like many other members of Russia’s political elite, was tainted by the political missteps and corruption that characterized Russia in the 1990s, though he was not implicated in any wrongdoing.
Mr. Nemtsov was outmaneuvered by Mr. Putin, a former K.G.B. agent, when it came time to replace Mr. Yeltsin as president.
Other politicians from the Yeltsin years went on to pursue lucrative business deals — sometimes taking advantage of their official positions — or dropped out of view when Mr. Putin assumed the presidency. Yet Mr. Nemtsov embraced his opposition role.
He was a leader of the Union of Right Forces, a party that tried in the early 2000s to counter Mr. Putin’s consolidation of power. His efforts drew the ire of Putin loyalists: The state-controlled news media and Kremlin supporters called Mr. Nemtsov corrupt and a traitor, and spread propaganda to undermine his popular appeal.
Later that decade, Mr. Nemtsov joined forces with Garry K. Kasparov, the former chess champion, to start a movement to stop Mr. Putin’s march toward authoritarianism. Harassed by the Kremlin and its supporters, Mr. Kasparov eventually chose self-imposed exile from Russia in 2013. Mr. Nemtsov continued his fight.
Risking harassment and arrest, he focused on government corruption, helping build a small opposition movement in 2011 that was quickly squashed when Mr. Putin returned to the presidency in 2012 after ceding the position to his handpicked stand-in, Dimitri A. Medvedev, for one term.
By the time Mr. Nemtsov was killed in 2015, he had lost the spotlight to Aleksei A. Navalny, an anticorruption blogger and opposition organizer. But Mr. Nemtsov remained active in Russian politics and had been organizing opposition to the war in Ukraine.
The gunman sentenced on Thursday, Zaur Dadayev, was convicted last month with his four accomplices. Prosecutors asked the court to sentence him to life in prison, but he was given 20 years.
Mr. Dadayev, who served as an officer in the security services of the Kremlin-backed Chechen leader, Ramzan A. Kadyrov, may have been the one who fatally shot Mr. Nemtsov, but the politician’s family and his supporters contend that the people who ordered the killing remain at large.
Investigators continue to collect evidence related to who ordered the killing, according to Svetlana Petrenko, a spokeswoman for Russia’s Investigative Committee. But Mr. Nemtsov’s supporters doubt that the authorities will bring the plotters to justice. Evidence in the case points to high-ranking government officials in Russia’s restive republic of Chechnya, led by Mr. Kadyrov, who is an ally of Mr. Putin.
Prosecutors have said that Mr. Dadayev and his accomplices killed Mr. Nemtsov after Ruslan Mukhudinov, the driver of a high-ranking Chechen military officer, promised to pay them $250,000. The prosecutors did not adequately explain why the driver would have that money or what would motivate him to organize the killing.
Mr. Mukhudinov was charged in absentia after fleeing to the United Arab Emirates. His boss, Ruslan Geremeyev, the deputy commander 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.
Investigators and allies of Mr. Nemtsov have long suspected that Mr. Kadyrov may have been involved in the killing, but it remains unclear why he might have wanted Mr. Nemtsov dead.
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 the Russian leader’s 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. The sentencing of the five Chechen men on Thursday is not likely to satisfy those who believe that the brazen death of Mr. Nemtsov was part of a larger plot. (Courtesy New York Times)