Although there is no confirmation that Marinova, who was 30, was murdered because of her television work, the case has led to calls from worldwide and domestic media freedom bodies who have urged a thorough investigation and linked it to Bulgaria's low ranking in media freedom in comparison with all other European Union countries.
The Committee to Protect Journalists said it was "shocked by the barbaric murder".
The Organization for Security and Co-operation in Europe's (OSCE) media freedom representative Harlem Desir condemned Marinova's killing on Twitter.
An investigative journalist was raped and murdered before being dumped by the Danube River.
The head of Ruse's prosecution office Georgi Georgiev however told a press conference on Sunday that nothing could be excluded at this stage.
"Her mobile phone, auto keys, glasses and part of her clothes were missing", Georgiev said, adding that prosecutors were probing all leads - both personal and linked to Marinova's job.
He also called on the Bulgarian government to do more about protecting the safety of journalists working in the country generally.
In the Bulgarian capital of Sofia, mourners gathered outside a church.
They did not offer any new details about the case after their meeting with the local authorities.
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In another high-profile case, journalist Dilyana Gaytandzhieva broke news previous year that large caches of U.S., Saudi, and Bulgarian weapons were secretly being shipped to Syria.
The European commission has 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.
News agency reports about the killing were carried on several prominent worldwide media, which cited other murders of journalists in European countries in 2018. "I will closely follow the investigation opened by the authorities".
The individual being held over the killing of Viktoria Marinova is a "Romanian citizen with a passport from Moldova", according to unconfirmed reports from Bulgaria's interior ministry broadcast on national radio. "There is no democracy without a free press. we expect a swift and thorough investigation to bring those responsible to justice", the European Commission said in a tweet.
A spokesman for the German Foreign Ministry said that it's imperative "that there's a fast investigation and that this frightful event will be illuminated as comprehensively as possible".
Giegold said, "First Malta, then Slovakia, now Bulgaria".
A journalist is killed on average every week around the world, according to figures compiled by RSF.
United Nations Secretary-General Antonio Guterres is paying "very close attention" to "a very worrying increase" in violence against journalists, especially women journalists, spokesman Stephane Dujarric said.