"I can confirm that her body has been found, which means that the suspicions are now about murder," said prosecutor Christer Sammens to Sveriges Radio.
The teenager was last seen in the early hours of Sunday morning. She was reported missing by her parents. Police have used dogs and helicopters in their search, and closed in on the lake after witnesses said they had seen three men in a boat acting strangely, news agency TT reports.
The lake is close to a farm which was cordoned off on Monday. Three young men had been questioned by police and were later formally arrested on suspicion of kidnapping. Now that a body has been formally identified, the prosecutor said that the suspicion has been upgraded to murder.