Russian journalist Arkady Babchenko has died after being shot near his home in Kiev, Ukrainian police say.
Mr Babchenko, 41, was found bleeding at the entrance to his block of flats by his wife and died in an ambulance.
After writing about a crashed Russian military plane in 2016, Mr Babchenko said he received death threats and left his home country.
He first moved to Prague and later to the Ukrainian capital. He was an outspoken critic of the Kremlin.
A former war correspondent, he worked in Kiev as a presenter on Ukraine's channel ATR TV.
What is known about the killing?
Speaking to the BBC, Ukrainian police spokesman Yaroslav Trakalo confirmed Mr Babchenko's death.
The journalist was reportedly shot several times in the back.
Kiev police chief Andriy Kryshchenko told local media they suspected Mr Babchenko was killed because of his "professional activities".
A criminal investigation is now under way.
Ukrainian lawmaker Anton Herashchenko said the journalist went out to buy some bread, and that the killer was waiting for him near his flat.
Ukrainian Prime Minister Volodymyr Groysman described Mr Babchenko as a "true friend of Ukraine, who was telling the world about Russian aggression".
"The killers should be punished," Mr Groysman said.
Russia's foreign ministry said in a statement: "We demand that the Ukrainian authorities make every effort to promptly investigate" the killing.
The ministry also offered condolences to Mr Babchenko's family and friends.
In his last Facebook post just hours before the attack, Mr Babchenko recalled how lucky he was four years ago - exactly to the day.
He wrote that he had planned to fly with Ukrainian soldiers on a helicopter in the war zone in Ukraine's east.
He was not allowed on board, because there was not enough space. The helicopter was shot down by pro-Russian rebels, leaving 14 people dead.
"I was lucky. A second birthday, it turns out", Mr Babchenko wrote.
Who was Arkady Babchenko?
A well-known Kremlin critic, he stood in unofficial elections organised by the opposition in 2012 and denounced Russia's actions in Syria and eastern Ukraine.
In December 2016, Mr Babchenko wrote a Facebook post (in Russian) about a crashed Tu-154 transport plane, which fell into the Black Sea while carrying a Red Army choir to Syria.
He claimed this Facebook post, in which he described Russia as an "aggressor", led to death threats and abuse from the Russian state, writing in a piece for The Guardian that this forced him to leave "a country I no longer feel safe in".
While studying law in Moscow aged 18, Mr Babchenko was conscripted into the Russian army and served during the Chechen wars from 1994-2000.
His memoir, One Soldier's War, chronicles his experiences in the conflict, which claimed the lives of tens of thousands on both sides.
He then became a journalist, working for a number of different outlets.
Mr Babchenko also wrote for the BBC, reporting from the scene of a Ukrainian army helicopter shot down in the east of the country in 2014.
Kiev has in recent years seen a number of deadly attacks on high-profile journalists and politicians. Most of them were vociferous critics of the Kremlin.