EU commissioners voiced "shock" and pledged help after the killing, at the weekend, of a Bulgarian journalist who reported on alleged fraud in European funds.

"Again a courageous journalist falls in the fight for truth and against corruption," European Commission vice-president Frans Timmermans said.

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Jourova offered help of EU agencies (Photo: European Commission)

"I will follow closely the investigation and offer help by EU agencies, if needed," EU justice commissioner Vera Jourova said.

They spoke after Viktoria Marinova, a 30-year old investigative journalist, was suffocated and beaten to death in a park in Ruse, in northeast Bulgaria, on Saturday (6 October).

She had also been raped.

She died after doing a TV report on alleged fraud involving EU tenders and Bulgarian infrastructure firm GP Group, according to the Balkan Investigative Reporting Network (BIRN), an EU-funded NGO in the region.

"Viktoria's death, the brutal manner in which she was killed, is an execution. It was meant to serve as an example, something like a warning," Asen Yordanov, the owner of Bulgarian news website Bivol.bg, which had worked with her on the EU fraud report, told the AFP news agency.

"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.

The Bulgarian interior minister, Mladen Marinov, said there was no evidence that the killing was linked to her work, however.

"It's about rape and murder," he said.

Bulgarian prime minister Boyko Borissov said: "It's only a matter of time before the murderer will be revealed. The best criminologists have been sent to Ruse, let's not hurry them. A large amount of DNA has been obtained".

The "horrendous murder", in Timmermans' words, comes after the killing of Jan Kuciak in Slovakia in February and of Daphne Caruana Galizia in Malta one year ago.

Kuciak had investigated alleged fraud in EU farming subsidies by an Italian organised crime group.

Caruana Galizia had investigated government corruption, including in sales of EU passports to wealthy foreigners.

The killings were part of a wider trend, according to Dimitrios Papadimoulis, a left-wing Greek MEP, who said "corruption is threatening democracy" in Europe.

Reporters Without Borders also said in a report earlier this year that Europe, a "traditionally safe environment for journalists" was becoming more "dangerous".

It put Bulgaria in 111th place - the worst in the EU - in its world press freedom ranking.

Jourova and MEPs have visited Malta and Slovakia to keep an eye on police enquiries.

The EU's joint police agency in The Hague, Europol, has also helped, but no one has been brought to justice yet.

Timmermans earlier led criticism of degrading standards of rule of law more broadly speaking in some central and eastern EU states.

They both appealed to Bulgarian authorities to catch Marinova's killers and shed light on what happened.

"Urgently call for a full and thorough investigation. Those responsible must be held to account," Harlem Desir, the media freedom envoy of the Organisation for Security and Cooperation in Europe, said.

"With great pain and insurmountable grief, the TVN team is experiencing the loss of our beloved colleague Viktoria Marinova and we pray for sympathy to the sorrow of her relatives and colleagues," TVN, the Bulgarian TV broadcaster where she had worked, said in a statement.