In the first episode, she interviewed two investigative reporters who were recently arrested while looking into corruption involving the misuse of European Union funds.
She had also been raped.
He said: "Viktoria's death, the brutal manner in which she was killed, is an execution".
Her body was recovered from the Danube river bank on Saturday, police said.
Police sources told Agence France-Presse that the crime did not immediately appear linked to her work.
The German government condemned her "brutal murder" and said it is imperative "that there's a fast investigation and that this terrible event will be illuminated as comprehensively as possible".
She was 30 years old.
The German government also sharply condemned the slaying, with the Foreign Ministry saying it was imperative "that there's a fast investigation and that this terrible event will be illuminated as comprehensively as possible".
In September, the two reporters had been detained by Bulgarian police.
In 2016, there were 1.1 homicides per 100,000 inhabitants in Bulgaria.
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"There is justified global skepticism about the Bulgarian state's commitment to press freedom and only a comprehensive and thorough investigation will assuage those doubts", he added. More than 10 years after joining the bloc, it has reported only limited progress in turning the tide against graft. Kuciak's reporting and his slaying brought down the Slovak government.
A 2016 report from the research organisation RAND Europe estimated that every year, the country loses $7 billion to $12 billion because of corruption - a fifth of its gross domestic product. The number of forbidden topics is growing all the time.
He says Dujarric said Guterres is paying "very close attention" to the trend and has raised the issue of violence against journalists with United Nations member states, both privately and publicly.
"It is so sophisticated and lacking in transparency that some people don't understand it and others don't believe it", he said.
In a July report on the state of media freedom in Bulgaria, Reporters Without Borders said, "Fortunately, no investigative journalists have been killed in Bulgaria for their work, but there are documented murder plans and attempts, cruel, execution-style, beatings, auto arson and bombs targeting investigative reporters".
Giegold said, "First Malta, then Slovakia, now Bulgaria".
In February, Slovakian journalist Jan Kuciak and his girlfriend Martina Kusnirova were shot dead. They called on Bulgarian authorities to identify, arrest and prosecute those responsible for Marinova's killing.
"I will closely follow the investigation opened by the authorities".
Marinova's death was quickly condemned by European leaders.
Her death comes after Daphne Caruana Galizia was killed by a auto bomb in Malta in October 2017, and reporter Ján Kuciak was shot dead at his home in Slovakia in February this year.