Professional athletes' use of performance- enhancing drugs has led hundreds of thousands of teenagers in the United States to turn to steroids that can cause "serious harm" to their development, the head of an investigation into baseball's own troubled past said Tuesday.

Former senator George Mitchell said the impact sports stars had as role models on steroid use among the young was the most "shocking" finding of his investigation into nearly two decades of illegal drug use by baseball players.

"The fact that hundreds of thousands of American youngsters are using steroids ought to be a wake-up call to every American, whether they're sports fans or baseball fans or not," Mitchell told a congressional hearing.

Mitchell led a months-long probe, at a cost of about 20 million dollars, that exposed widespread use of illegal performance-enhancing drugs in baseball since the early 1990s. The problem was widely known by owners, managers and baseball's authorities, and Mitchell described a "collective failure to recognize the problem as it emerged and to deal with it early on."

Members of the House Oversight Committee criticized the league for a "code of silence" that prevented the issue from emerging during its peak in the last decade. Baseball banned steroids in 1991, but testing was only agreed to by the players' union in 2002 and has since been strengthened.

Among dozens of players implicated in Mitchell's report released last month was Roger Clemens, whose 20-year career as a pitcher is regarded as one of the finest in baseball's history. Clemens, who won a record seven Cy Young awards as the best pitcher of the year, has strongly denied he ever took steroids and will appear before Congress in February.

But Mitchell, who among other high-profile jobs helped mediate an end to the conflict in Northern Ireland, said the focus of action should be on ending steroid use, rather than exposing and punishing players that were implicated in the past.

He related his advice to the "difficult" process of releasing prisoners in Northern Ireland that had committed "brutal criminal acts" for what they believed were patriotic causes.

"I learned then that sometimes you have to turn the page and look to the future," Mitchell said. "I believe that everyone involved should be trying to bring this troubling chapter in baseball's history to a close."

But Mitchell warned that aside from giving players a competitive advantage, the most dangerous impact of steroid use was its role in "encouraging" young people to follow the lead of their sports role models.

"This goes far beyond baseball ... Baseball players are not the only persons who are role models for young people," he said. "It's a broad societal issue of which baseball is only a part."