Prosecutors in the Balkan country said that the body of 30-year-old Viktoria Marinova was found in a park in Ruse on Saturday.
"We call for police protection to be provided for the TVN journalists who [also] worked with Viktoria Marinova on the same sensitive report, pending the outcome of the investigation", Reporters Without Borders, a French NGO, said.
That appearance was her first time anchoring the show on Ruse-based TV station TVN - a popular channel in northeastern Bulgaria.
Dimitar Stoyanov, of the Bivol website, and Attila Biro, from the Romanian Rise Project, have alleged a ring of consultancy firms...
Daphne Caruana Galizia, Malta's best-known investigative reporter, was killed by a vehicle bomb near her home auto in October last year and Slovak journalist Jan Kuciak and his fiancée Martina Kusnirova were shot dead in February this year.
Mladen Marinov, Bulgaria's interior minister, described the murder as "exceptionally brutal", according to local news outlets.
The organisation says investigative journalists in the country "are often subjected to pressure, from mere warnings to intimidation and physical assaults on themselves or their property".
Chief Public Prosecutor Sotir Tsatsarov arrived in Russian Federation and announced Monday that authorities had no new leads on the motive for the slaying.
He said his site had received "operational information that hits against our journalists working on this story are being discussed and prepared".
Condolences poured in on Facebook for Marinova who leaves behind a small child.
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Bulgarian officials emphasised that there was no evidence yet to connect Marinova's killing to her work. "We expect a swift and thorough investigation to bring those responsible to justice", the European Commission, the European Union's executive said in a tweet.
A journalist is killed on average every week around the world, according to figures compiled by RSF.
Daphne Caruana Galizia was killed by a auto bomb in October 2017 while investigating corruption among Malta's ruling elite.
"All leads are being looked at" in the crime investigation including possible links to her professional activity, chief prosecutor Sotir Tsatsarov said Monday.
"We are running out of live reporters to report on the dead ones", Drew Sullivan, editor and co-founder of the Organized Crime and Corruption Reporting Project (OCCRP), warned of the danger to journalists.
The murder was similarly condemned by the European and International Federation of Journalists, both of which said they were deeply shocked by the murder.
He quoted from a speech European Commission President Jean-Claude Juncker made last month: "Too many of our journalists are intimidated, attacked or even murdered". Two of her guests were arrested in September, the BBC reports.
"It is about rape and murder", he said. In Ruse, mourners, some tearful, placed candles, her portrait and roses- the national flower- at the foot of a monument. Slovakian investigative journalist Jan Kuciak and his girlfriend were shot to death in February.
The horrific indent is the third murder of a journalist in the European Union amid concerns over press freedom.
Jourova and MEPs have visited Malta and Slovakia to keep an eye on police enquiries, but no one has been brought to justice yet.