The "forest boy" who has stumped German investigators for nine months has been identified as a 20-year-old Dutchman, police in Berlin say.

Investigators said Robin van Helsum's stepmother recognized a picture of the young man that was released to the news media earlier this week, ABC News reported. His father died in February after he ran away.

When he turned up in Berlin last year, van Helsum said he did not know his last name. He claimed to have been living in a forest with his father since his mother's death in a car crash five year's earlier.

In fact, police say, he had been living in Hengelo, a town near the German border. His parents reported him missing in September 2011, but Dutch police did not pursue the case because he was not a minor and did not appear to be at risk.

Van Helsum has been living in housing for teenagers in Berlin. Authorities say his hoax could be costly for him.

"This is no joke anymore," Berlin police spokesman Michael Maass told the German newspaper Die Welt. "He made right fools of us. The costs could come down to him."

Police say it would be as much as $25,000 to cover expenses related to the Forest Boy.

Van Helsum told police his father had died accidentally, and he had followed his emergency instructions to walk north. Investigators said they were suspicious because he did not look like he had been living rough for years, and his tent and other equipment were comparatively new, but they were unable to find any teens who had been reported missing who matched his description. (c) UPI