"The shocking news about the murder of Victoria Marinova, an investigative journalist, seemingly linked to her reports on investigations into alleged corruption involving European Union funds, urges us to take immediate action at all levels and with all legal means", Moraes wrote.
CPJ reported that the two reporters were detained by Bulgarian police in September.
Marinova, a 30-year-old investigative reporter, was found in the northern Bulgaria city of Ruse, close to the border with Romania.
Police said the man was a Romanian citizen of Ukrainian origin and would be detained for 24 hours as they check his alibi.
The body of Viktoria Marinova, 30, was discovered near the Danube on Saturday in Ruse.
Ruse Regional Prosecutor Georgi Georgiev told state media that Marinova's mobile phone, vehicle keys, glasses and part of her clothing were missing when her body was found.
"It is about rape and murder", Interior Minister Mladen Marinov said.
Marinova was a director of TVN, a TV station in Ruse, and a TV presenter for two investigative programs.
Exposing corruption: Bulgarian TV journalist Viktoria Marinova.
"We are in shock". Margaritis Schinas, 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". "In no way, under any form, never have we received any threats - aimed at her or the television".
Journalists' groups and foreign officials expressed shock.
"I see deliberate attempts to marginalize this killing and manipulate the public opinion that Viktoria was a victim of a random attack or sexual manslaughter", Assen Yordanov said, adding that Marinova was the only TV journalist who had agreed to host his website's journalists to discuss the investigation.
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.
In a July report on the state of media freedom in Bulgaria, Reporters Without Borders said, "Fortunately, no investigative journalists have been killed in Bulgaria for their work, but there are documented murder plans and attempts, cruel, execution-style, beatings, vehicle arson and bombs targeting investigative reporters".
Reporters Without Borders ranks Bulgaria last in the European Union for press freedom.
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