Marinova, 30, had reported on an investigation into corruption of European Union funds shortly before she was raped and brutally murdered in the northern town of Ruse on the Danube river, according to Bulgarian authorities.
The suspect was a Romanian citizen.
But Bulgaria's interior minister Mladen Marinov said there was no evidence to suggest the killing was linked to Ms Marinova's work.
Local people held a candlelight vigil overnight and many left flowers to commemorate Marinova, who left a seven-year-old daughter.
The Balkan nation, which joined the European Union in 2007, was ranked 71st on Transparency International's corruption list past year. He said the country's top murder investigators had been sent to Ruse to work on the case.
Bulgarian media reported that the park where her body was found is adjacent to a psychiatric facility and that authorities were investigating whether a patient could have attacked Marinova, reported The Washington Post.
The murder case has shocked fellow journalists and sparked worldwide condemnation. "Again a courageous journalist falls in the fight for truth and against corruption", the EU Commission's vice president Frans Timmermans tweeted late Sunday.
Marinova was a director of TVN, a TV station in Ruse, and a TV presenter for two investigative programs.
Marinova is the third journalist to be murdered in Europe in the past 12 months after Jan Kuciak in Slovakia in February and Daphne Caruana Galizia in Malta in October 2017. The journalists, Attila Biro and Dimitar Stoyanov, had been detained in September for their work.
"I see deliberate attempts to marginalize this killing and manipulate the public opinion that Viktoria was a victim of a random attack or sexual manslaughter", Assen Yordanov said, adding that Marinova was the only TV journalist who had agreed to host his website's journalists to discuss the investigation.
She had recently been working on corruption allegations against businessmen and politicians. "It was meant to serve as an example, something like a warning".
Marinova's killing was widely-condemned on Twitter by fellow journalists and watchdog groups.
Bulgarian police, however, said they are considering all possible scenarios and examining possible links to both her personal and professional life.
"Investigative journalists are being systematically removed", she said.