Company boss Georgiev was found shot dead outside his Sofia home
The EU has called on Bulgaria to take "urgent action" to fight organised crime after two prominent figures were shot dead in 24 hours.
The European Commission said the killings in the capital, Sofia, were not just another statistic.
Georgi Stoev, a former gang member who wrote nine books on the Bulgarian mafia, was shot in the head as he was leaving a cafe. He died of his wounds.
Earlier, Borislav Georgiev, head of a nuclear repair firm, was shot dead.
His company, Atomenergoremont, maintains reactors at the Kozlodui nuclear plant.
Next week, EU experts will travel to Bulgaria before the European Commission issues a report in July on the country's progress in fighting high-level corruption and contract killings.
The EU imposed a special corruption monitoring scheme on Bulgaria and its neighbour Romania when they joined the EU in January 2007.
There is growing pressure to recommend unprecedented sanctions, the BBC's European affairs correspondent Oana Lungescu reports from Brussels.
The EU could cut some of the funds for Bulgaria or refuse to recognise arrest warrants and criminal judgments issued there.
"Urgent action is required in the area of fighting organised crime in Bulgaria," said European Commission spokesman Mark Gray on Tuesday.
"Unfortunately these shootings have continued to take place on a regular basis over the last couple of years and without successful prosecution."
Bulgaria has seen some 150 mafia-style killings since the fall of communism in 1989, and not a single conviction.
The EU has already frozen some of its infrastructure subsidies for Bulgaria.
Bulgarian Prime Minister Sergey Stanishev publicly pledged that his country would do its utmost to prosecute those responsible for the latest attacks, but diplomats say he is involved in a power struggle with the interior minister, Rumen Petkov.