Tsankov was gunned down in a busy district of Bulgaria's capital
Boris "Bobbie" Tsankov, a prominent crime journalist who reported on the mafia in Bulgaria, has been killed by gunmen in the capital, Sofia.
The 30-year-old, who was also a popular radio host, was attacked on a crowded street in the city centre, police said.
Two men who were with him were also shot and critically wounded, before the gunmen escaped on foot.
In 2008, Georgi Stoev, the author of several books on Bulgarian organised crime, was killed in a similar attack.
Months later, Bulgaria lost access to more than 500m euros (£430m) of EU funding for failing to deal with corruption and organised crime.
Responding to Tuesday's killing, European Commission spokesperson Mark Gray said "shootings continue to be a problem that needs to be urgently addressed in Bulgaria".
"Any shooting is unacceptable and we hope that the Bulgarian authorities will bring those that have perpetrated this act to justice as quickly as possible."
Since his book The Secrets of the Mobsters was published last November, Mr Tsankov said he had received numerous death threats.
The book, based on his contacts with underworld bosses such as the drug dealer Anton Miltenov, who was shot dead in 2005, alleged links between mafia figures and businessmen.