Margaritis Schinas, a spokesman for European Commission President Jean-Claude Juncker, said Monday that the commission expected "a swift and thorough investigation. that will bring those responsible to justice and clarify whether this attack was linked to her work".
Bulgaria's prosecutor general, Sotir Tsatsarov, confirmed the arrest of Severin Krassimirov, a 21-year-old Bulgarian citizen, but gave no further details of his location.
CPJ reported that the two reporters were detained by Bulgarian police in September.
A Bulgarian man has been detained in Germany over the rape and murder of television journalist Viktoria Marinova, officials said on Wednesday, as Prime Minister Boyko Borissov said journalists in Bulgaria had total freedom to write and report.
The 30-year-old journalist told him she was working on an investigation but didn't feel anxious about her safety and had not received threats.
Marinov said the suspect had a criminal record for scrap-metal theft. He said there was no information that she had been threatened.
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Interior Minister Mladen Marinov and Bulgaria's leading organized crime investigator, Ivaylo Spiridonov, are part of the investigating team.
The brutal killing of 30-year-old Viktoria Marinova - who presented a current affairs talk programme called "Detector" for the small TVN television channel - has shocked the country and drawn worldwide condemnation. It also ranks 111 out of 180 in terms of press freedom, the lowest in the European Union, according to Reporters Without Borders (RSF), a French organization devoted to the protection of journalists.
bTV, another Bulgarian broadcaster, claimed that police had found the journalist's mobile phone in the suspect's apartment in the northern town of Ruse. The interior ministry said he left Bulgaria early Saturday afternoon, crossing the bridge at Ruse over the Danube into Romania. A Bulgarian investigative online media site went further, calling for an independent global inquiry and saying corruption could compromise an investigation by Bulgarian law enforcement.
She was a presenter on a current affairs talk programme called Detector, which had recently aired an interview with two Bulgarian investigative journalists exploring alleged fraud involving European Union funds and prominent businessmen and politicians.
While Marinova didn't appear to have been closely involved in the fraud investigation, her show touched on a sensitive subject in Bulgaria, where corruption is endemic.
Led by the anticorruption group BOETS, the demonstrators have pledged to continue protesting daily until the journalist's killer is found.