Belarus-born investigative journalist Sheremet was killed in central Kyiv on Wednesday while driving to work to front a radio talk show. He also worked at the investigative website Ukrayinska Pravda.

The news website said the car blown up belonged to its founding editor Olena Prytula.

Images showed the charred car in the middle of a street. It was not clear whether the explosive device was detonated by timer or remote control.

Ukrainian interior minister Khatiya Dekanoidze said she would personally supervise the investigation."We are looking at all theories," she said.

Began career in Belarus

Belarussian-born Sheremet, who began his career clashing with authoritarian president Alexander Lukashenko, moved in 1999 to Russia where he worked over 12 years for several Russian broadcasters as a television journalist.

He quit Russia's OTR channel in 2014 in protest at what he saw as the Kremlin's propaganda-style coverage of Ukrainia's "Maidan" protests and later accused Russia of illegally annexing Crimea.

Sheremet's friend Nemtsov had been working on a report examining Russia's military role in the Ukraine crisis when he was shot dead near the Kremlin last year.

Sheremet led tributes at his memorial service.

Poroshenko calls in FBI

Reacting to Wednesday's killing, Ukrainian President Petro Poroshenko said he had asked the US Federal Bureau of Investigation to join the murder investigation in the interests of "maximum transparency."

At Ukrainska Pravda, Sheremet was executive director while also fronting his own morning show on Vesti Radio. He was married and had two sons and a daughter.

In 2002, Sheremet won the a prize for journalism and democracy from the Organization for Security and Cooperation in Europe (OSCE) for his reporting on human rights violations in Belarus.

The OSCE called Wednesday for efforts to improve the safety of journalists in Ukraine.

No longer comfortable visiting Moscow

The news agency Reuters said Sheremet had told it last October that he no longer felt comfortable visiting Moscow - despite gaining Russian citizenship in 2000.

"I'm threatened often and given hints. Every time I go to Moscow, it's like I'm in a minefield," he told Reuters in an interview.

Sheremet also said Ukraine needed a strong, independent media to counter outlets controlled by the country's powerful business tycoons.

ipj/kl (Reuters, AFP)