Who was outspoken Putin critic Boris Nemtsov?

The brash, sharp-tongued Russian opposition leader shot and killed late Friday was an outspoken critic of Putin who had been threatened for his views.

Boris Nemtsov, 55, a former first deputy prime minister, was shot and killed shortly before midnight Friday by an unknown gunman who jumped from a white car, fired around seven shots and then sped off.

As co-chair of the Parnas political party and one of the leaders of the anti-Kremlin Solidarnost movement in Russia, Nemtsov was particularly outspoken regarding the leadership in the Kremlin, especially Russian President Vladimir Putin.

"I'm afraid Putin will kill me," he told the Sobesednik blog two weeks ago in an interview, citing his activism. Regarding the Russian president, he added: "I couldn't dislike him more."

Nemtsov had been working on a report proving Russia's involvement in the Ukrainian conflict. He had received anonymous threats in the past several weeks, said Ilya Yashin, one of the leaders of the Parnas Party.

Moscow and Kiev have been locked in a dangerous political and territorial battle since the Ukrainian opposition last year toppled president Viktor Yanukovych, who was pro-Russian. Pro-Moscow rebels have been battling Ukrainian troops for control of eastern Ukraine. Putin has denied arming the rebels or fighting with them, despite reports of Russian troops and armaments across the border.

Speaking on radio just a few hours before his death, Nemtsov accused Putin of plunging Russia into crisis by his "mad, aggressive and deadly policy of war against Ukraine," the Associated Press reported.

Nemtsov believed that Putin wanted revenge, fearing that a pro-Europe Ukraine posed a threat to his power.

"He lies in revenge for Ukraine's revolution, when Ukrainians took to the streets and dethroned the corrupt thief president Yanukovych. He is afraid it could be repeated in Russia. And, besides, he thinks if Ukraine is successful on the European path, it is a threat to his own power," he told the U.S.-government-backed Voice of America during a September interview.

Nemtsov had been scheduled to appear at a Sunday opposition march protesting Russian involvement in Ukraine. Organizers canceled the march, instead planning a gathering to mourn him.

In the early 2000s, Nemtsov founded a liberal opposition party but it failed at the polls. He briefly dropped out of politics, focusing on business and aiding opposition forces in Ukraine. He also wrote about corruption in Russia and the enrichment of Putin's rich and powerful inner circle, known as the Oligarchs.

Nemtsov supported Ukraine's Orange Revolution in 2004 and became an economic adviser for Ukrainian President Viktor Yushchenko, who ousted the presidential candidate backed by Yanukovych. In 2008, Nemtsov helped create the Solidarity movement that deposed Yanukovych.

Nemtsov, a nuclear scientist and environmentalist, was long in the forefront of political upheaval in Russia as one of the earlier young economic reformers. He won the post of governor of the Nizhny Novgorod Oblast in 1991 at age 32.

Home to military industries, the region became a showcase for foreign investment after the fall of communism, and the media-savvy Nemtsov — who spoke fluent English — quickly became one of the country's most prominent and influential politicians.

But the economic crisis of 1998 cost him his job, tarnished his reputation and dashed hopes that then-president Boris Yeltsin would anoint him as his successor. Instead, the presidential scepter was passed to Putin.

Contributing: Anna Arutunyan