Bulgaria's chief prosecutor, Sotir Tsatsarov, said the suspect was already wanted in connection with another rape and murder.
In a case that shocked Bulgaria and drew worldwide condemnation, mourners bearing flowers and wreaths flocked to the northern town of Ruse to pay their respects to Marinova, whose body was discovered near a jogging path next to the Danube last Saturday.
A 20-year-old Bulgarian suspect, named as Severin Krasimirov, was tracked down and arrested in the northern German town of Stade, near Hamburg, on Tuesday.
Hundreds of people were expected to attend a funeral service for Marinova in her hometown of Ruse on Friday.
Ruse Mayor Plamen Stoilov told reporters that "the brutal slaying has cast a shadow over the peaceful city".
But the man said "he did not want to kill the victim and denied raping her", the public prosecutor's office in the northern German city Celle said in a statement.
On her last television show, on 30 September, Ms 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.
Prosecutors say the suspect confessed to being under the influence of alcohol and drugs when he got into an argument with a woman he did not know in a park.
A German court said Friday the suspect would "soon" be sent back to Bulgaria and the Bulgarian justice ministry added later that the extradition would take place "next week at the earliest". While Marinova didn't appear to have been closely involved in the fraud investigation, her show touched on a sensitive subject in Bulgaria, where corruption is endemic, and there was widespread speculation that she may have been targeted for her work.
Ms Marinova's murder has triggered a debate over the extent of press freedom in the Balkan country and the rights of journalists to pursue investigative reporting.
Marinov said investigators had spoken to Marinova's family and friends and that there was "no apparent link to her work" as a journalist. "Further, the US government condemns violence against women in any form".