The murder of prominent Putin critic Boris Nemstov in a gangland-style killing steps from the Kremlin came just weeks after the dissident told a magazine his mother worried the Russian leader would have him bumped off for his outspokenness.

"'When will you stop cursing Putin? He'll kill you for that.' She was completely serious," Nemstov told Sobsesdnik earlier this month, according to the Wall Street Journal. The paper added that the former Deputy Prime Minister under Russian president Boris Yeltsin expressed some worry about his safety but not as much as his mother.

The Boston Globe reported that Nemstov also told the magazine in reference to his 86-year-old mom, "She is truly scared that he (Putin) could kill me soon for all of my statements, both in real life and on social networks. This is not a joke; she is a smart person."

During the chat with the magazine, Nemstov said that if he were really very fearful he probably wouldn't have led an opposition party.

Nemtsov, 55, was gunned down in a drive-by shooting Friday near midnight as he walked on a bridge near the Kremlin with a female companion.

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Russia’s top investigative body said it would explore several possible motives for the murder including an attempt to destabilize the state, Islamic extremism, the Ukraine conflict and his personal life.

The statement from Investigative Committee did not address the likeliest possibility, as seen by many of Nemstov’s supporters—that he was executed for being one of President Vladimir Putin's most adamant and visible critics.

The finger-pointing began as investigators converged on the scene where Nemstov’s body was lying in the street after he had been shot four times in the back by a gunman in a car.

“Devastated to hear of the brutal murder of my long-time opposition colleague Boris Nemtsov. Shot 4 times, once for each child he leaves,” fellow Putin opposition leader and human rights activist Garry Kasparov tweeted.

Another Kasparov tweet said, “Do not say stupidities about how this is bad for Putin, or that he wouldn't do this. Do not presume to tell a dictator his business.” A third said, “Dictators do not ask why when it comes to taking human life, only why not. Putin has oceans of blood on his hands already and is still there.”

Nemtsov was working on a report presenting evidence that he believed proved Russia's direct involvement in the separatist rebellion that has raged in eastern Ukraine since April. Moscow denies backing the rebels with troops and sophisticated weapons.

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

The killing came the day before he was to help lead a rally protesting Russia's actions in the Ukraine crisis and the economic crisis at home.

After his death, organizers canceled the rally and instead called for a demonstration to mourn him on Sunday in central Moscow. The city gave quick approval for that gathering, in contrast to its usual slow and grudging permission for opposition rallies.

The Investigative Committee said it was looking into whether Nemtsov had been killed as a "sacrificial victim for those who do not shun any method for achieving their political goals," a suggestion echoing the comments by Putin's spokesman and other Russian politicians that the attack was a "provocation" against the state.

It also said it was considering whether there was "personal enmity" toward him in his domestic life. State-controlled TV and Kremlin-friendly media outlets on Saturday gave considerable attention to Nemtsov's companion, identifying her as a Ukrainian model 30 years his junior and showing photos of her in alluring poses. The Investigative Committee said the pair were headed for Nemtsov's apartment.

The statement also said it was investigating whether the killing was connected to the Ukraine conflict, where Russia-backed separatists have been fighting Ukrainian forces since last April, or whether there was a connection to Islamic extremism.

Nemtsov's death hit other members of the anti-Putin opposition hard. The march of mourning on Sunday could serve to galvanize the beleaguered and marginalized opposition, or it could prove to be a brief catharsis after which emotions dissipate.

Through the morning Saturday, hundreds of people came to the site of Nemtsov's death to lay flowers at a make-shift memorial.

Putin ordered Russia's top law enforcement chiefs to personally oversee the investigation of Nemtsov's killing.

"Putin noted that this cruel murder has all the makings of a contract hit and is extremely provocative," presidential spokesman Dmitry Peskov said in remarks carried by Russian news agencies.

President Barack Obama said the Russian people "lost lost one of the most dedicated and eloquent defenders of their rights."

German Chancellor Angela Merkel praised Nemtsov's courage in criticizing Kremlin policies, and urged Putin to insure that the killers are brought to justice, her spokesman Steffen Seibert said.

Former Soviet leader Mikhail Gorbachev echoed the suggestion that the killing was a provocation. "It's an attempt to push the situation into complications, maybe even to destabilizing the situation in the country," he was quoted as saying by the Interfax news agency.

Communist Party leader Gennady Zyuganov agreed. "It's a provocation; for big fires, sacrificial figures are necessary," Interfax quoted him as saying.

Nemtsov frequently assailed the government's inefficiency, rampant corruption and Ukraine policy.

Kasyanov, the former prime minister, said he was shocked.

"In the 21st century, a leader of the opposition is being demonstratively shot just outside the walls of the Kremlin!".

"This is a monstrous tragedy and a loss for us all," Alexei Navalny, Russia's most prominent opposition figure, said on his Facebook page. He is currently serving a 15-day jail sentence for handing out leaflets without authorization.

"The country is rolling into the abyss," Kasyanov, the former prime minister, told reporters as Nemtsov's body, placed in a plastic bag, was removed on a rainy and cold night, as the Kremlin bells chimed nearby.

Nemtsov once was seen as a possible successor to Boris Yeltsin, Russia's first elected president.

Nemtsov was widely liked for his good humor, larger-than-life character and quick wit, but he and other top opposition figures long have been purged from state television and steadily marginalized by the Kremlin.

The Associated Press contributed to this report.