On her last TV show, on September 30, Marinova introduced two journalists who were investigating suspected corruption involving European Union funds and said her own show, "Detector", on local television station NTV, would carry out similar investigations.
Bulgarian interior minister Mladen Marinov initially denied that Marinova's murder was linked to her reporting, and said there was no evidence she had been threatened. "The Romanian citizen is to be released", Teodor Atanasov, the head of Ruse regional police, told the media on Tuesday.
Marinova was raped and strangled, and her body found on Saturday near the Danube River after she highlighted possible government corruption.
Prosecutors said that she had been raped, suffocated and beaten with such force that her face was unrecognisable.
Bulgarian prosecutors announced on Monday night that they have started a probe into the alleged embezzlement of European Union funds by GP Group, a construction company that the two investigative reporters were researching.
Ms Marinova is the third European journalist to be killed in the past year.
A Bulgarian investigative online media site has gone further, calling for an independent global inquiry, saying a Bulgarian investigation could be compromised by corrupt Bulgarian officials.
The minister said investigators had spoken to Marinova's family and friends and "there is no apparent link to her work".
Marinova's death also drew worldwide calls for a rapid and resolute action, with UN Secretary-General Antonio Guterres urging governments to ensure "accountability" for crimes against journalists such as the Bulgarian journalist's "grisly murder and rape".
Speaking under the condition of anonymity, a TVN journalist told Agence-France Presse, "We are in shock. In no way, under any form, never have we received any threats - aimed at her or the television".
He said: "Viktoria's death, the brutal manner in which she was killed, is an execution". Harlem Desir, the media freedom representative for the Organization for Security and Cooperation in Europe, demanded a "full and thorough investigation" of Marinova's slaying. Caruana Galizia was killed in a vehicle bombing in October in Malta and Jan Kuciak was murdered in Slovakia in February.