She is the third journalist killed in the European Union in the past year.
He said it evidence suggested it was "a spontaneous attack, not premeditated".
Chief Prosecutor Tsatsarov told the news conference that he could not say at this stage if the murder was linked to Marinova's work as a journalist. "It is about rape and murder", he said said.
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.
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Marinova was killed by blows on the head and suffocation, the authorities said, adding that prosecutors were probing all leads - both personal and linked to Marinova's job. Her professional identity as a host on TVN, a local television program, focused on investigative journalism and immediately stoked fears about retributions against journalists exposing corruption schemes, especially in Eastern Europe.
"We have detained a man". The journalists, Attila Biro and Dimitar Stoyanov, had been detained in September for their work. Authorities are working to identify witnesses and potential motives for her murder.
The European Commission has called for a "swift and thorough investigation" into Ms Marinova's killing.
Through a United Nations spokesperson, Secretary-General António Guterres called on "all governments to strengthen press freedom and ensure there is accountability for crimes committed against journalists".
Marinova is the third journalist to have suffered a violent death in the European Union over the past 12 months, after Jan Kuciak in Slovakia in February and Daphne Caruana Galizia in Malta in October a year ago.
Azoulay said: "Attacks on journalists erode the fundamental human right to freedom of expression and its corollaries, press freedom and free access to information". 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.