According to a report in VG, the man decided to admit the murder after visiting his mother's grave. Prior to her death, he had told her what he had done and he said that his mother's voice came from the beyond and told him to go to police.
The man went to police in November and told them that he had killed Nilsen. The two had been living together in an Oslo flat and he said that he killed her in the midst of an argument and then hid her corpse in Finnskogen forest.
Police, however, have found no trace of the body and have now gone public with the case in a plea for help.
“The police have carried out a thorough investigation without the woman being found and we are now asking for the public's help to solve the case,” police attorney Sturla Henriksbø told news agency NTB.
Nilsen had no close family and was thus never reported missing in 1993, police said.
“Both her and her roommate where part of [Olso's] drug environment. The investigation uncovered that there has not been a single trace of Nilsen in public registries since 1993. She has never been reported missing. The murder is thought to have occurred in September 1993,” Henriksbø said.
He added that police have searched Finnskogen “several times without making a find” but said that the man who stepped forward to claim the murder was unable to pinpoint where he hid the body.
Police charged the man with murder after his November confession but said that certain inconsistencies in his story mean that investigators cannot be totally certain that Nilsen is dead.
“The core of the accused's explanation is believable since she hasn't left a trace since 1993. But there are also several contradictions in his explanation, including in how he killed her,” Henriksbø told VG.
The timeline is also unclear, as the 62-year-old claims to have killed Nilsen in 1992 but police found records indicating that she appeared in court in March 1993.
Even with the man's confession, police said they may not be able to convict him without definitive proof that Nilsen is in fact dead. They are therefore asking the public to come forward with anything that might help them find the body.
“We hope that there might be someone who may have seen clothing, jewellery, bones or something else [in the forest]. That could help us limit which areas we need to search,” Henriksbø told VG.