Viktoria Marinova's body was found on Saturday in a park by the River Danube in the northern town of Ruse.
Marinova worked for a small local TV station called TVN where she presented two investigative programs.
Her final show was about Attila Biro, a Romanian whistle-blower who leads the Rise Project Romania, probing into corruption.
And even some of Marinova's fellow journalists were not convinced that she was killed because of her reporting. She'd been beaten with such force that she was unrecognizable, according to the Federation of European Journalists.
"It's about rape and murder", Interior Minister Mladen Marinov said. The two reporters she spoke to were briefly detained after the show aired.
Chief Prosecutor Sotir Tsatsarov said: 'We do not exclude that it was a random attack, we do not exclude that it was a premeditated attempt on her life'.
United Nations Secretary-General Antonio Guterres is paying "very close attention" to "a very worrying increase" in violence against journalists, especially women journalists, spokesman Stephane Dujarric said.
Several hundred people, many in tears, attended a vigil in Ruse's central square, lighting candles and laying flowers in front of a portrait of Marinova.
A journalists' group has called for an investigation into the "brutal" slaying of a Bulgarian television reporter and presenter, but warned against speculating about the motive for her killing. In the first episode, she interviewed two investigative reporters who were recently arrested while looking into corruption involving the misuse of European Union funds.
Bulgaria is ranked 111 out of 180 countries in the Reporters Without Borders World Press Freedom index.
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.
It is not known if Marinova's murder is linked to her journalism work.
"While the motive for this horrific crime remains unclear, we emphasize that Bulgarian authorities must consider all angles, including the possibility that Ms Marinova was killed in connection with her work", the International Press Institute's Deputy Director Scott Griffen said in a statement.
"We are in shock".
"It's about rape and murder", he said.
The crime has sparked global outcry, with condemnation from the OSCE and the Committee to Protect Journalists.
Bulgaria tumbled down to 111th place in the annual RSF media freedom ranking in 2018 - lower than any other European Union member state. He tweeted: "Again a courageous journalist falls in the fight for truth and against corruption".
She is the third reporter in the E.U.to be murdered in the past year. Then in February, Jan Kuciak and his fiancee were shot dead in Slovakia in a case tied to his reports on tax evasion.
The Guardian reported that Bivol.bg owner Asen Yordanov had told AFP that his media had received credible information that its journalists were in danger of being assaulted because of the investigation that also appeared on Marinova's show.