"I am shocked by the horrific murder of investigative journalist Viktoria Marinova in Bulgaria", Harlem Desir, the Organisation for Security and Cooperation in Europe's representative on freedom of the media, said.
The country's chief prosecutor, Sotir Tsatsarov, said he could not yet say whether Ms Marinova's death was linked to her work.
The Bulgarian government said it had no evidence that Marinova's murder was linked to her work.
Authorities discovered the body of 30-year-old Viktoria Marinova on Saturday near the river in the northern Bulgarian town.
"It is about rape and murder", Interior Minister Mladen Marinov said, according to The Guardian.
Worldwide, at least 48 journalists have been killed doing their work in 2018, according to the Committee to Protect Journalists, an advocacy group that does an annual count of detained and killed journalists.
The Post reported on Tuesday night, citing a person familiar with the investigation, that before Khashoggi's disappearance, USA intelligence intercepted communications of Saudi officials discussing a plan to capture him.
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.
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Several months ago, she became the host of an investigative program called Detector.
The two had been investigating alleged fraud with European Union funds linked to big businessmen and politicians when they were detained by armed Bulgarian police for more than seven hours in mid-September as they followed up on a tip that crucial documents were being burned in a field outside the capital, Sofia.
Among the most high-profile recent cases were Malta's anti-corruption blogger Daphne Caruana Galizia who died in a vehicle bombing outside her home in October 2017, and top Slovak reporter Jan Kuciak who was shot dead with his fiancee at their home in February.
The Center for the Study of Democracy, based in the capital, Sofia, outlined in a report past year a portrait of a state so riddled with graft that 1 in 5 adults, or 1.3 million people, were thought to have taken part in a corrupt transaction, such as paying or receiving a bribe.
The 30-year-old journalist told him she was working on an investigation but didn't feel anxious about her safety and had not received threats.
Bulgaria ranked 111 out of 180 countries in the Reporters Without Borders world press freedom index this year, lower than any other European Union member. "Those responsible should be brought to justice immediately by the Bulgarian authorities", he said in a tweet.
In October 2017, Daphne Caruana Galizia was killed by a vehicle bomb while investigating corruption among Malta's ruling elite.
Jourova and MEPs have visited Malta and Slovakia to keep an eye on police enquiries, but no one has been brought to justice yet.
But they said that, at least for now, the killing does not appear linked to her work as a journalist.