Record-breaking Olympic swimmer Michael Phelps issued an apology on Sunday after a British newspaper published a photograph purportedly showing him smoking marijuana.
In a statement, Phelps, 23, the winner of a record eight gold medals at the Beijing Olympics last year, said he had engaged in "behavior which was regrettable and demonstrated bad judgement."
US Anti-Doping Agency chief executive Travis Tygart told Reuters the news was disappointing but stressed that smoking marijuana out of competition was not an anti-doping matter at this point.
The US Olympic Committee and USA Swimming also expressed disappointment, given the swimmer is a role model for young people.
The News of the World said the photograph of Phelps was taken at a house party being held by students at the University of South Carolina in Columbia last November.
Phelps, in his statement, promised there would be no repeat.
"I'm 23-years-old, and despite the successes I have had in the pool, I acted in a youthful and inappropriate way, not in a manner that people have come to expect from me," Phelps said.
"For this, I am sorry. I promise my fans and the public - it will not happen again."