The brazen killing of Russian opposition politician Boris Nemtsov in central Moscow on Friday sent shockwaves across Russia and beyond.
A spokesman for Russian President Vladimir Putin was quick to condemn the shooting, which happened just outside the walls of the Kremlin.
"Putin noted that this cruel murder has all the makings of a contract hit and is extremely provocative," Dmitry Peskov told reporters, noting that the president had ordered Russia's top police chiefs to personally oversee the murder investigation.
But there was immediate pressure from inside Russia and out for the inquiry to be more thorough than those that have come before.
US President Barack Obama called on Russia's government to launch "a prompt, impartial, and transparent" investigation to ensure that "those responsible for this vicious killing are brought to justice".
Mr Obama, who met Nemtsov in Moscow in 2009, said he had admired the former deputy prime minister's "courageous dedication to the struggle against corruption in Russia".
Meanwhile German Chancellor Angela Merkel condemned the "cowardly murder". A statement by her office demanded that Mr Putin "assure himself that this assassination is elucidated and that its perpetrators are held accountable".
A UK Foreign Office spokeswoman said "we are shocked and saddened by the news".
'Friend of Ukraine'
Nemtsov had been working on a report into Russia's involvement in the conflict in eastern Ukraine in the weeks before his death.
Russia denies supplying Ukrainian rebels with heavy weapons and troops, but the West and the government in Kiev say they have evidence to the contrary. It appears Nemtsov believed he too had acquired such evidence.
Arseniy Yatsenyuk, Ukraine's prime minister, tweeted (in Ukrainian) shortly after Nemtsov's death was confirmed, saying: "Patriot of Russia, at same time a friend of Ukraine. This is what will remain in our memory of Boris Nemtsov. RIP".
Ukrainian President Petro Poroshenko called Nemtsov a "bridge between Ukraine and Russia".
"The murderers' shot has destroyed it. I think it is not by accident," Mr Poroshenko wrote on Facebook (in Ukrainian).
Fellow Russian opposition politician Garry Kasparov said he was "devastated" by news of the killing.
"He always believed Russia could change from inside without violence; after 2012 I disagreed," Mr Kasparov wrote.
"When we argued, Boris would tell me I was too hasty, that in Russia you had to live a long time to see change. Now he'll never see it. RIP."
He blamed Nemtsov's killing on Mr Putin's "atmosphere of hatred and violence, abroad and in Russia," saying bloodshed is "the prerequisite to show loyalty".
Nemtsov's lawyer Vadim Prokhorov said the politician had received several death threats on social media in recent months, but said authorities did not take them seriously.
Mikhail Kasyanov, a former Russian prime minister who is now in opposition, said he was shocked.
"In the 21st century, a leader of the opposition is being demonstratively shot just outside the walls of the Kremlin!" he told reporters at the scene of the murder. "The country is rolling into the abyss."
Opposition activist Ilya Yashin said he had no doubt that the killing was politically motivated.
"Boris Nemtsov was a stark opposition leader who criticized the most important state officials in our country," Mr Yashin said. "As we have seen, such criticism in Russia is dangerous for one's life."
Michael McFaul, US ambassador to Russia from 2012-2014, said the shooting was one of the "most shocking things that I can remember happening in Russia for a long, long time".
"100 meters from the Kremlin and none of those militsiya [police] working around there could stop the murderers?" he tweeted.
Opposition activists were due to hold a rally in Moscow on Sunday but said they would be staging a funeral march through the city instead.
"We are in a new political reality," one of the rally organisers, Leonid Volkov, said on Twitter (in Russian).