Police in Germany have arrested a suspect in the rape and killing of Bulgarian journalist Viktoria Marinova, officials said.
The country's chief prosecutor, Sotir Tsatsarov, said he could not yet say whether Ms Marinova's death was linked to her work.
The motivation behind her killing remains unknown, and Bulgarian authorities say they have yet to establish a link between her death in the northeastern city of Ruse and her work as a journalist.
Marinova's final show was a program about Attila Biro, an investigative journalist with the Rise Project Romania and a colleague from the Bulgarian investigative site Bivol.bg, Dimitar Stoyanov.
"At this stage, we do not believe that the murder is linked" to Marinova's work, Tsatsarov said.
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. He declined to answer reporters' question about whether there was signs she had been sexually assaulted. She had been raped and died of blows to the head and suffocation, according to investigators.
Marinova's murder has drawn worldwide condemnation and calls from media watchdogs for a comprehensive investigation.
In a statement released by CPJ, Marinova had interviewed two journalists who had been looking into allegations of fraud involving the European Union and a global investigating reporting platform. A Maltese investigative reporter was killed in a auto bomb in October a year ago, and a Slovak journalist was shot dead in February.
United Nations spokesman Stephane Dujarric said Monday that Guterres was "very concerned" about Marinova's slaying and awaited the conclusions of the investigation into her killing.
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The Center for the Study of Democracy, based in the capital, Sofia, outlined in a report a year ago a portrait of a state so riddled with graft that 1 in 5 adults, or 1.3 million people, were thought to have taken part in a corrupt transaction, such as paying or receiving a bribe.
In February, 27-year-old Kuciak and his fiancée, Martina Kusnirova, also 27, were shot dead after he began digging into connections between top government officials and organised crime.
In October 2017, Daphne Caruana Galizia, a Maltese reporter who specialised in government corruption and money laundering, was killed by a auto bomb near her home.
Bulgarian authorities are requesting his extradition, she said.
Bulgaria tumbled to 111th place in the annual RSF media freedom ranking in 2018 - the lowest among European Union member states.
Margaritis Schinas, spokesman for European Commission President Jean-Claude Juncker, said Monday 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".
Europe has seen the steepest decline in World Press Freedom Index regional rankings over the past year: Malta is now ranked 65th, down by 18 points, and Slovakia 27th, down by 10.
Bulgarian media reported Monday that the park where Marinova was killed is adjacent to a psychiatric facility and authorities were investigating whether a patient could have been Marinova's attacker.
News agency reports about the killing were carried on several prominent global media, which cited other murders of journalists in European countries in 2018.