The body of 30-year-old Viktoria Marinova was found on Saturday close to a jogging path by the Danube, Ruse prosecutor Georgy Georgiev said.
"All leads are being looked at" in the crime investigation including possible links to her professional activity, chief prosecutor Sotir Tsatsarov said Monday.
Ms Marinova was planning a report about a separate case of alleged embezzlement when she was killed.
Last October, prominent Maltese journalist Daphne Caruana Galizia, who exposed the Maltese prime minister's offshore dealings in the Panama Papers scandal, died in a auto bomb attack.
Bulgarian Prime Minister Boyko Borisov said evidence including DNA had been discovered at the crime scene and that "it is just a matter of time before the perpetrator is found".
Mahoney says that it is too early to say whether Marinova's killing falls into that category.
The TVN channel expressed shock over the killing of its colleague and board member, but declined to comment further, saying it would give investigators time to do their job first.
"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.
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Marinova's murder has drawn worldwide condemnation and calls from media watchdogs for a comprehensive investigation.
In the first episode, she interviewed two investigative reporters who were recently arrested while looking into corruption involving the misuse of European Union funds. Attila Biro, editor of the Romanian investigative website Rise Project, and Dimitar Stoyanov, of the Bulgarian site Bivol, were arrested in the Sofia suburb of Pernik on 14 September and held for six hours.
A friend of Marinova, Todor Gechev, told reporters from Bulgarian National Television (BNT) that he met with her on Wednesday.
A journalist is killed on average every week around the world, according to figures compiled by RSF.
He also called on the Bulgarian government to do more about protecting the safety of journalists working in the country generally.
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".
"As security - rather than the protection of fundamental rights and freedoms - becomes the number one priority of governments worldwide, broadly-written security laws have been twisted to silence journalists", wrote Index on Censorship's Jodie Ginsburg a year ago.