The European commission and German government have urged Bulgarian authorities to bring to justice those responsible for the brutal killing of the journalist Viktoria Marinova, who had been reporting on alleged corruption in one of the EU’s newest member states.

The commission vice-president, Frans Timmermans, said he was “shocked by the horrendous murder” of Marinova. He tweeted: “Again a courageous journalist falls in the fight for truth and against corruption. Those responsible should be brought to justice immediately by the Bulgarian authorities.”

The German government condemned the “brutal and dreadful murder” of Marinova and said it was imperative that “there is a fast investigation and that this horrible event will be illuminated as comprehensively as possible.”

Marinova, a 30-year-old Bulgarian TV journalist, was found dead in a park in the north Bulgarian city of Ruse on Saturday. She had been raped and died of blows to the head and suffocation, according to investigators.

She is the third journalist to be murdered in the European Union in less than a year. Daphne Caruana Galizia was killed by a car bomb in October 2017 while investigating corruption among Malta’s ruling elite. In February, Ján Kuciak, a Slovakian investigative reporter, 27, and his fiancée, Martina Kušnírová, were shot dead in their home, 40 miles from Bratislava.

The Bulgarian government said it had no evidence that Marinova’s murder was linked to her work. The Bulgarian prime minister, Boyko Borisov, said: “The best criminologists were sent to Ruse, let’s not press them. A large amount of DNA had been obtained.”

Police and prosecutors said they were working on three possible motives – that Marinova was a victim of a random attack by a person with a mental disorder, a planned attack or a crime linked to her personal life. “We do not exclude that it was a random attack, we do not exclude that it was a premeditated attempt on her life,” chief prosecutor Sotir Tsatsarov told reporters.

The Association of European Journalists’ Bulgaria chapter called for an investigation in the murder, but warned against speculation. “As long as there are no solid and proven facts, the speculations that the incident is connected to the freedom of expression in the country are unacceptable,” it said.

Sven Giegold, an influential German Green MEP, urged the European parliament to send a delegation to Bulgaria, as it did in the case of Caruana-Galizia and Kuciak. “The cruel murder of Marinova concerns the whole of Europe,” he said. “First Malta, then Slovakia, now Bulgaria: It is unacceptable that journalists are murdered again in Europe.”

Since joining the EU, Bulgaria has tumbled down the rankings for media freedom, dropping from 51st place in 2007 to 111th in 2018. The Balkan state has the lowest score on press freedom in the EU – but also falls below non-EU states including Ukraine andothers in the Balkans, such as Kosovo, Montenegro and Serbia.

Reporters Without Borders, which compiles the World Press Freedom index, said in its 2018 survey that “physical attacks and death threats against journalists by criminal groups are especially common in Bulgaria.” It described corruption and collusion between media, politicians and oligarchs in Bulgaria as widespread.

In the week before her murder, Marinova presented a TV show that broadcast interviews with reporters from the Bivol website on alleged fraud involving EU funds linked to businessmen and politicians.

The EU’s anti-fraud office, Olaf, said it had begun looking into potential misuse of EU funds in Bulgaria but had not yet opened a full investigation.

“Olaf is aware of allegations concerning the possible misuse of EU funds in Bulgaria that have been brought to light by journalists in recent weeks,” a spokesperson said. “It is only after such an initial assessment, which is currently ongoing, that Olaf decides whether or not to open an investigation,” the agency said.

The Organization for Security and Co-operation in Europe’s representative for media freedom, Harlem Désir, said there was an increasing trend of attacks against female journalists.

“I will closely follow the investigation opened by the authorities. I urge them to swiftly identify and bring to justice those responsible and to clearly determine whether this attack was linked to her work,” he said.