AP
According to one Moscow-based reporter, Nemtsov was attacked "200 [meters] from the Kremlin walls."
Nemtsov served as a deputy prime minister and a regional governor in the 1990s under previous Russian president Boris Yeltsin, and advised Ukrainian president Viktor Yushchenko after Ukraine's "Orange Revolution" in 2004.
In the 1990s, as governor of Russia's Nizhny-Novgorod region, Nemtsov, a former physicist, helped implement free-market reforms. His dissident streak stretched all the way back to the existence of the Soviet Union: according to the New York Times, he met with Andrei Sakharov when the Nobel Peace Prize-winning nuclear physicist and prominent anti-communist figure was still living in internal exile.
More recently, he emerged as one of the leading domestic critics of Russian president Vladimir Putin.
In 2000, Nemtsov viewed Putin as "Russia's best bet" for continuing the political and economic reforms Yeltsin initiated. But Nemtsov, who was one of Russia's highest-profile liberals, grew into one of Putin's most dogged opponents. In February of 2011, Nemtsov, who led investigations into Russian government corruption, told an interviewer that "Putin has created a thoroughly rotten system and he believes it is possible to build this mighty foundation known as Russia on cynicism, thievery and lies."
He was arrested for organizing protests multiple times over the years, and planned to lead a march against the war in Ukraine scheduled for this coming Sunday.
Nemtsov's death is a significant development, according to Hannah Thoburn, a Eurasia analyst at the Foreign Policy Initiative. If anyone connected to the Russian government or Russian president Vladimir Putin was involved, Nemtsov's death might have been intended to intimidate Russia's beleaguered opposition.
"People like Nemtsov have been allowed to stay around on the scene because they were useful, because Russia needed to keep up pretenses, because they could be reasonably trusted to be happy with their small, pro-Western followings and stay otherwise out of the way of Putin's will," Thoburn explained to Business Insider by email.
"Now, we don't know if Nemtsov was mixed up in some dirty business, but my initial instinct says that this was deliberate and purposeful and meant to send a message to all of the other opposition sorts to shut up. If they aren't afraid to kill Nemtsov, who is very well known in the West, then they won't hesitate to kill some activist in Perm.
"This means that the rules have changed. Everyone needs to get with the program or get out."
This isn't the only recent blow to Russia's opposition. On February 19, prominent opposition and anti-corruption activist Alexei Navalny, who is currently under house arrest, was sentenced to 15 days in jail.