The man - a Romanian - was being held for 24 hours, Atanasov said.
The body of 30-year-old Viktoria Marinova was found on Saturday near a pedestrian alley in an area with heavy vegetation, Bulgarian state media reported.
"Her death was caused by blows to the head and suffocation, and her mobile phone, vehicle keys, glasses and some of her clothing were missing", Ruse regional prosecutor Georgy Georgiev said. The first episode aired on September 30 and featured two investigative journalists looking into alleged fraud by businessmen and politicians involving European Union funds, reports the BBC.
The Balkan nation joined the European Union in 2007 and was ranked 71st on Transparency International's corruption list past year.
She was the director of TVN, a TV station in Ruse, and an anchor of two investigative programs.
Marinova's death also drew global calls for a rapid and resolute action, with UN Secretary-General Antonio Guterres urging governments to ensure "accountability" for crimes against journalists such as the Bulgarian journalist's "grisly murder and rape".
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Atanassov told reporters that the man had not been named a suspect in the murder.
But Bivol.bg owner Asen Yordanov said he had received credible information his journalists were in danger of being assaulted because of the investigation that featured on Ms Marinova's show.
She had recently been working on corruption allegations against businessmen and politicians.
He said: "Viktoria's death, the brutal manner in which she was killed, is an execution".
Journalists' groups and foreign officials expressed shock. "It was meant to serve as an example, something like a warning".
Harlem Desir, media freedom representative for the Organization for Security and Cooperation in Europe's (OSCE) condemned the killing on Twitter, saying that he would closely follow the investigation.