Bulgarian police have questioned a man in connection with the killing of investigative journalist Viktoria Marinova, whose body was found in a park in her hometown of Ruse four days ago.

Key points: Journalist was found raped and murdered near the Danube River

Journalist was found raped and murdered near the Danube River She had been reporting on the alleged misuse of EU funds

She had been reporting on the alleged misuse of EU funds Reports said a man they had questioned would be released "very shortly"

Police said they would release and will not charge the man, who wasn't identified, after he was taken into custody in connection with the rape and murder.

The man would be freed "very shortly," said Teodor Atanassov, chief police officer of the northern town of Ruse. He declined to provide further details on the ongoing investigation.

Police are investigating the rape and slaying of Marinova, 30, who was strangled and her body found near the Danube River after she highlighted possible Government corruption.

Police earlier said the man would be detained for 24 hours as they checked his alibi, but that they were yet to determine a main suspect.

Marinova, 30, hosted a show last month featuring two investigative journalists who were detained for their work on suspected fraud involving EU funds.

Prosecutors said late on Monday that they had opened an investigation into the suspected misuse of European Union funds.

Interior Minister Mladen Marinov and Bulgaria's leading organised crime investigator, Ivaylo Spiridonov, are part of the investigating team.

Viktoria Marinova is the third journalist to be murdered in the EU in the past year. ( Facebook: Viktoria Marinova )

While she did not appear to have been closely involved in the fraud investigation, her show touched on a sensitive subject in Bulgaria, where corruption is endemic.

The Balkan nation joined the EU in 2007 and was ranked 71st on Transparency International's corruption list last year.

Joining the bloc opened an enormous spigot of possible new EU funding for Bulgarian infrastructure projects or other programs designed to bring the nation up to EU standards.

Journalist groups and European leaders expressed shock at Marinova's murder.

Margaritis Schinas, spokesman for European Commission president Jean-Claude Juncker, said on 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."

The German Government also sharply condemned the slaying, with the Foreign Ministry saying it was imperative "that there's a fast investigation and that this horrible event will be illuminated as comprehensively as possible".

ABC/wires