A spokesman for the Kremlin said Putin believes the "cruel murder ... has every sign of being a contract killing, which has a solely provocative nature," according to the Interfax news agency.
While police in Russia search for a suspect, Nemstov joins a growing list of Putin's critics who have died or have been killed in recent years:
A court convicted five men of involvement in her murder last year, but Russia's Investigative Committee has not yet determined who ordered the killing.
He died in November 2006, 21 days after he was poisoned.
British police named two Russian men as the prime suspects but Russia has refused to extradite them.
The accountant and auditor was imprisoned in November 2008 after exposing massive corruption. He was found dead in his cell a year later.
Authorities ruled his death a heart attack, but supporters claimed Magnitsky was beaten to death.
The leading Russian human-rights lawyer had represented Politkovskaya and other anti-Chechen-War dissidents. He was shot to death outside in Kremlin in January 2009.
Also killed in the shooting was Anastasia Baburova, a freelance journalist.
A Russian nationalist extremist was sentenced to life in prison for the killings.
Russian investigators said in 2010 that two brothers who were members of an Islamic militant group killed Estemirova, who headed Memorial group's Chechen branch. Memorial said DNA evidence showed that the two men didn't commit the crime.
The popular Russian actor renounced two awards he had "received from Putin's hands" because he was "ashamed."
“I've had enough of all this czar-state stuff," he wrote in a blog post.
Devotchenko was found dead in his apartment in November 2014. Russian investigators originally told reporters his death "was of a criminal nature," but they later said they found he cut his hand and hit his head after drinking too much, and bled out, according to The Telegraph.
The Associated Press contributed to this story.