The body of Viktoria Marinova, 30, was found in a park in the northern town of Ruse.
Bulgarian police are investigating the rape and slaying of a television reporter and presenter whose body was dumped near the Danube River.
"With enormous pain and insurmountable grief the team of TVN television is experiencing the loss of our beloved colleague, Victoria Marinova", TVN said in a statement. She hosted the recently-launched 'Detector' program, and its last episode covered the investigation by Bivol website, claiming to expose a corruption scheme inside EU-funded projects.
Ruse regional prosecutor Georgy Georgiev said Ms Marinova had suffered blows to the head and was suffocated in the fatal attack, adding that her "mobile phone, vehicle keys, glasses and part of her clothes were missing".
The Organization for Security and Cooperation in Europe, an intergovernmental organization, called for a "thorough investigation" of her rape and murder, noting "a trend of increased attacks against female journalists".
"The country has a bad image with regard to press freedom, but it's possible that there is no link to this case", said Svetoslav Terziev, opposition media analyst and journalism teacher.
The pair were briefly detained by police, drawing condemnation from Reporters Without Borders (RSF).
Chief Prosecutor Sotir Tsatsarov said: 'We do not exclude that it was a random attack, we do not exclude that it was a premeditated attempt on her life'.
The Bulgaria-based Association of European Journalists said reporters from small regional and local media are under particular pressure and even face threats from local businessmen and politicians, often leading to self-censorship.More news: Google Pixel Slate renders appear to reveal design, detachable keyboard and stylus
The killings were part of a wider trend, according to Dimitrios Papadimoulis, a left-wing Greek MEP, who said "corruption is threatening democracy" in Europe.
Marinov told reporters on Monday in Ruse, Bulgaria's main river port: 'We are working on all possible motives and we do not exclude any'.
But Mr Marinov said there was no evidence to suggest the murder was related to Ms Marinova's work and there was no information that she had been threatened.
Dujarric told reporters at United Nations headquarters in NY "there's been a very worrying increase of violence, sexual and otherwise, that's particularly targeting women journalists".
"It's about rape and murder", he said. Swedish freelance journalist Kim Wall was murdered in a gruesome case in Denmark past year by Danish inventor Peter Madsen. In Ruse, mourners, some tearful, placed candles, her portrait and roses- the national flower- at the foot of a monument.
Marinova is the third journalist to have died in the European Union over the past year, and the fourth since the start of 2017. That's lower than any other member of the EU. Slovak journalist Jan Kuciak was shot dead along with his fiancee in February after he reported on corrupt ties between Slovak officials and Italian mobsters.
The horrific indent is the third murder of a journalist in the European Union amid concerns over press freedom.
Reporters Without Borders warned in 2017 about a suspected murder plot against a Bulgarian publisher, Georgi Ezekiev. "I will closely follow the investigation opened by the authorities".
Jourova and MEPs have visited Malta and Slovakia to keep an eye on police enquiries, but no one has been brought to justice yet.