The 41-year-old was reportedly shot in the back in his home
There has been condemnation of the killing of Russian journalist Arkady Babchenko.
He was shot dead at his apartment in Kiev, Ukraine on Tuesday.
The 41-year-old left his home country last year, saying he had received thousands of threats.
According to the Moscow-based independent newspaper Novaya Gazeta, where he had worked until 2011 as a war correspondent, Babchenko was shot in the back in the entrance of his Kiev home by an unknown assailant or assailants.
His wife found him and called an ambulance, but he died of on the way to the hospital.
The Committee to Protect Journalists (CPJ) Europe and Central Asia Program Coordinator, Nina Ognianova, said: "We call on Ukrainian authorities to thoroughly, effectively, and independently investigate the killing of Arkady Babchenko, and to find all those responsible for this brutal, silencing crime.
"Babchenko was well known for his critical journalism, and authorities must consider his murder as an attack on press freedom."
At least one journalist has been murdered in Ukraine in the past two years in direct retaliation for their work.
Pavel Sheremet was killed in Kiev on July 20th, 2016 when an explosive device detonated under the car he was driving.
CPJ say no one has been convicted of the crime.
Babchenko was veteran of the First Chechen War who had specialised in covering conflict after his discharge from the military.
He covered such conflicts as the 2008 Russia-Georgia war over South Ossetia, and the 2010 inter-ethnic violence in southern Kyrgyzstan.
He was a sharp critic of Russia's annexation of Crimea and its intervention in east Ukraine, according to press reports.
He left Russia for Prague in February 2017 after receiving threats, and moved to Ukraine in August with his family.