It is not clear if Marinova's murder was related to her journalistic work.
EU commissioners voiced "shock" and pledged help after the killing, at the weekend, of a Bulgarian journalist who reported on alleged fraud in European funds.
The European Commission and the German government have joined a number of global journalism organizations in condemning - and calling for an independent investigation into - the killing of Victoria Marinova, a local TV journalist in the town of Ruse, northern Bulgaria. The best criminologists have been sent to Ruse, let's not hurry them.
Also on the minds of press freedom advocates is the disappearance of Jamal Khashoggi, a prominent Saudi critic and Washington Post contributor, who vanished last week after entering the Saudi consulate in Istanbul. We expect a swift and thorough investigation to bring those responsible to justice'.
"Again a courageous journalist falls in the fight for truth and against corruption", said Frans Timmermans, vice president of the European Commission.
"It is about rape and murder", Interior Minister Mladen Marinov said, a viewpoint that was shared by Prime Minister Boiko Borisov.
Viktoria Marinova had recently become the anchor of a talk show focused on investigative reporting.
He also said all the DNA data they have will be provided to the German authorities.
The first episode of the show on September 30 broadcast interviews with investigative journalists Dimitar Stoyanov from the Bivol.bg website and Attila Biro from the Romanian Rise Project, about an investigation into alleged fraud involving European Union funds linked to big businessmen and politicians. Her first and final programme in this slot, which went out on September 30, covered an investigation by "Bivol" into alleged embezzlement of European Union funds earmarked for infrastructure projects.
A Romanian man arrested over Ms Marinova's murder on Tuesday has been released without charge, police said.
The police said it is investigating whether the murder Marinova with her professional activities.
Chief Prosecutor Sotir Tsatsarov said: 'We do not exclude that it was a random attack, we do not exclude that it was a premeditated attempt on her life'.
The Commission urged Bulgaria to conduct a rapid investigation and the USA embassy said it was shocked by the "horrific murder".
In October 2017, Daphne Caruana Galiza, a Maltese journalist known for exposing high level corruption, was murdered by a bomb placed on her vehicle.
The crime has sparked global outcry, with the OSCE's media freedom representative, Harlem Desir, calling for "a full and thorough investigation".
"Those responsible must be held to account", one of their representatives wrote on social media.
"In certain countries there is just not enough enforcement of the law", Robert Mahoney, deputy executive director of the Committee to Protect Journalists, told NPR.