MOSCOW - For the tens of thousands bearing flowers and tying black ribbons to railings in honor of slain Russian opposition politician Boris Nemtsov, the solemn march through the Moscow drizzle on Sunday was a time for silence, not slogans.
The marchers occasionally broke into chants of "Russia without Putin," or "Say no to war," but often the only sound was the steady thwack of police helicopters overhead or the hum of police boats patrolling the shores of the Moscow River.
The mourners on Sunday marched to the bridge near the Kremlin where Nemtsov was gunned down shortly before midnight Friday.
Nadezhda Tolokonnikova, a member of the once-jailed Russian punk protest group "Pussy Riot," tweeted a photo of a large crowd near the bridge with an "I am not afraid" sign.
The mood was somber, with a heavy police presence. Ilya Yashin, a friend and fellow opposition leader, said he hoped the killing would not frighten people.
"Essentially it is an act of terror. It is a political murder aimed at frightening the population, or the part of the population that supported Nemtsov and did not agree with the government," Yashin told The Associated Press. "I hope we won't get scared, that we will continue what Boris was doing."
While the killing of Nemtsov has shaken the Russian opposition, which sees the Kremlin as responsible, it is unclear whether his death will be enough to invigorate the beleaguered movement. Despite the Ukraine conflict and Russia's economic crisis, support for President Vladimir Putin has been above 80 percent in the past year.
Since mass anti-Putin protests brought hundreds of thousands to the streets of Moscow in 2011 and 2012, Putin has marginalized and intimidated his political opponents, jailing some, driving others into exile, and ramping up fines and potential jail time for those detained at protests.
Whoever was responsible for the slaying, the signal it sends to Putin's foes is that if Nemtsov can be killed for his political activism then no one is safe. As a former deputy prime minister and longtime politician, he retained strong ties among Russia's political and business elite.
Russia's federal investigative agency said it was looking into several possible motives for his killing.
The first possibility, the Investigative Committee said, was that the murder was aimed at destabilizing the political situation in Russia and Nemtsov was a "sacrificial victim for those who do not shun any method for achieving their political goals."
This suggestion echoed comments by Putin's spokesman and other Russian politicians that the attack was a "provocation" against the state.
Former CBS News Moscow bureau chief Beth Knobel told CBS News Radio there is some evidence to support this argument.
"It kind of hurts Putin to have somebody in the opposition killed because Nemtsov is going to be a martyr," Knobel said.
Additionally, the investigative agency 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.
Opposition activists had planned a protest rally on Sunday, which the city demanded they hold in a suburban neighborhood. After Nemtsov's death, they called instead for a demonstration to mourn him in central Moscow. The city gave its quick approval.
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.
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 Obama said the Russian people "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.
The widow of murdered Russian spy Alexander Litvinenko thinks the Russian government may have been involved in the murder of lNemtsov.
Marina Litvinenko told BBC Radio on Sunday she thinks Nemtsov's slaying Friday night in Moscow seems to have been the Russian government's way of silencing critics of President Vladimir Putin, but offered no proof.
She said the Russian government has become particularly aggressive since the Ukraine crisis began. She called Nemtsov's death "absolutely devastating."