Marinova was found near the Danube river promenade in the town of Ruse on Saturday, after going out for a coffee and a jog, the regional prosecution said on Sunday at a press conference. They are also investigating whether her death is linked to her work.
United Nations Secretary-General Antonio Guterres is calling on governments to ensure "accountability" for crimes against journalists such as the "grisly murder and rape." - including Bulgarian TV reporter Viktoria Marinova.
The Interior Ministry said late Monday that prosecutors had opened an investigation into GP Group, a large private Bulgarian building company alleged to have misused the European Union money, and froze 14 million euros ($16 million) of its assets.
Frans Timmermans, commission vice-president, said he was "shocked by the horrendous murder of Victoria [sic] Marinova". "There is no democracy without a free press. we expect a swift and thorough investigation to bring those responsible to justice", the European Commission said in a tweet.
It comes after Daphne Caruana Galizia, an investigative journalist from Malta, was killed in October previous year after a bomb blew up her vehicle and Slovak journalist Jan Kuciak was shot dead in February.
Authorities in Slovakia say they have narrowed the list of those suspected of ordering the killing but declined to offer any details while the investigation was in progress, noting that the plot may have also involved targeting a high-ranking prosecutor involved in prosecuting corruption.
RSF called for other journalists at the station to have police protection while the investigation takes place.
Bulgarian police questioned a Romanian man about the murder of TV journalist Viktoria Marinova on Tuesday but released him without charge and the country remains under worldwide pressure to solve the crime.
"It is about rape and murder", he said.
Bulgaria was under pressure Monday to find the killer of a television journalist whose brutal murder sparked global condemnation, as mourners thronged candlelight vigils in the capital Sofia and the northern town of Ruse where she was killed. She had been beaten, raped and strangled, according to local media.
Vigils in Marinova's memory were held Monday evening in Ruse, the capital Sofia and other cities.
Bulgarian Prime Minister Boyko Borissov told reporters: "I am convinced it is a matter of time before the murder would be revealed".
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. Slovak journalist Jan Kuciak was shot dead along with his fiancee in an attack linked to his reporting on tax evasion in February this year.
On the advice of legal counsel, journalists at TVN were not commenting to their colleagues.
A journalist is killed on average every week around the world, according to figures compiled by Reporters Without Borders (RSF).