The body of 30-year-old Viktoria Marinova, identified only by her initials, was found in a park in Ruse on Saturday, according to prosecutors in the Balkan country.
Interior Minister, Mladen Marinov, said: ”It is about rape and murder.”
At the time of her death, the journalist had been involved in covering an investigation by a group of Bulgarian journalists into companies involved in EU-funded infrastructure projects administered by local authorities, according to local reports.
But Mr Marinov said there was no evidence to suggest the murder was related to Ms Marinova’s work and there was no information that she had been threatened.
Bulgarian Prime Minister Boyko Borissov: “I am convinced it is a matter of time before the murder would be revealed.
“The best criminologists were sent to Ruse, let’s not press them.
“A large amount of DNA had been obtained.”
Police are expected to disclose more details about the murder on Monday.
Georgy Georgiev, Ruse regional prosecutor, added: “Her death was caused by blows to the head and suffocation, and her mobile phone, car keys, glasses and some of her clothing were missing.”
Ms Marinova, who was a board member of the Ruse-based TV station TVN – a popular TV channels in north eastern Bulgaria – is the third journalist to have been murdered across the continent in a year.
TVN said in a statement: ”With great pain and insurmountable grief the TVN’s team is experiencing the loss of our beloved colleague Victoria Marinova and we pray for sympathy to the sorrow of her relatives and colleagues.”
It comes after Daphne Caruana Galizia, an investigative journalist from Malta, was killed in October last year after a bomb blew up her car and Slovak journalist Jan Kuciak was shot dead in February.
Bulgaria is ranked 111 out of 180 countries in the Reporters Without Borders world press freedom index – lower than any other EU member and also lower than other countries in the western Balkans, some of which are candidates for EU membership.
In October 2017 hundreds of Bulgarian journalists protested in Sofia against threats from Deputy Prime Minister Valeri Simeonov against the nation’s biggest broadcasters after he accused mainstream media of leading a “massive smear campaign” against him.