The man and woman, who were reportedly divorced but living together in a farmhouse in Höxter, were arrested last week.
Investigators believe that they lured a 41-year-old woman to their home through a personal ad in a newspaper, before holding her against her will for weeks in their cellar.
They are believed to have physically assaulted her so badly that they eventually called ambulance after her condition rapidly deteriorated. But within hours, she had died in hospital.
Now, police sources have told DPA that the couple have admitted to another murder.
Prosecutors in Paderborn said on Monday that they could not confirm nor deny this report, but that there was evidence of other victims.
"We have concrete evidence that we are investigating," a spokesperson for the prosecutors' office said, adding that officials plan to send sniffer dogs to the property.
The 46-year-old man reportedly often tried to meet women using personal ads.
The 47-year-old woman was also brought back to the house on Sunday in order for her to "explain a few things," investigators said.
Officials are expected to give a press conference on Tuesday to reveal the latest progress in the investigations.