Award-winning journalist Pavel Sheremet has been killed in a car bomb explosion in Ukraine's capital, Kiev.
Sheremet, a Belarusian-born journalist with popular news website Ukrayinska Pravda, was killed on Wednesday morning while driving a car owned by his partner, Olena Prytula, who is the publication's owner.
He was on his way to work to anchor a talk show on Vesti Radio when the bomb went off.
The 44-year-old was known for his outspoken criticism of the leadership of his home country, Belarus. He was also a close friend of Russian opposition leader Boris Nemtsov, who was shot dead close to the Kremlin last year.
'Looking at all theories'
Ukranian President Petro Poroshenko offered his condolences to Sheremet's friends and family, adding that he had instructed law-enforcement agencies to conduct "a speedy investigation into this crime".
An improvised explosive device was planted underneath the car, Zoryan Shkiryak, adviser to the Ukrainian interior minister, said in a Facebook post.
It was unclear whether the device had been set off by a remote control or a timer, Shkiryak added.
The bomb is believed to have contained up to the equivalent of 600 grammes of TNT.
Interior Minister Khatiya Dekanoidze said in televised comments at the scene of the crime that she would monitor the investigation herself.
"We are looking at all theories," she said, adding that solving the murder was "very important" as it was "a matter of honour" for the Kiev police.
Anton Gerashchenko, a member of parliament and adviser to the interior ministry, referred to the killing of Sheremet as a "cynical murder", adding that it could be used to "to destabilise the internal political situation in Ukraine".
In 2002, Sheremet won a journalism prize from the Organization for Security and Co-operation in Europe (OSCE) for his reports on human rights violations in Belarus, including the disappearances of opposition politicians and journalists.
In 2013, Sheremet began working for a new Russian broadcaster, Public TV and Radio (OTR), but quit in 2014 in protest at what he saw as the Kremlin's propaganda in coverage of the Ukrainian crisis.
He openly accused Russia of illegally annexing Crimea and supporting pro-Moscow separatists battling government forces in Ukraine's east.
Sheremet had told the Reuters news agency in October that he no longer felt comfortable visiting Moscow.
"I'm threatened often and given hints. Every time I go to Moscow, it's like I'm in a minefield," he said in an interview.
On Wednesday, the OSCE called for action to address the safety of journalists in Ukraine.