The officer began working for Warner Brothers job several months after the preliminary investigation was completed. The same police officer is scheduled to appear as a witness in the forthcoming Pirate Bay trial, newspaper Sydsvenskan reports.
Defence lawyer Peter Althin said he would be looking into the matter.
"The question is how long this was under consideration. If it was under consideration at the time of the investigation then it is a scandal," he told Sydsvenskan.
Althin is representing Peter Sunde, one of four men charged charged with being an accessory to breaking copyright law.
"This is a judicial scandal. Talk about a conflict of interests," Sunde told the newspaper.
If the police officer is found to have entered into discussions with Warner Brothers before the end of the investigation, which took a year and a half to complete, it is possible that the prosecution will have to scrap its findings and start again, said Althin.