Lt. Chris Cowan said Tuesday that Richland County sheriff's investigators are gathering more information about the photo, which showed the swimmer inhaling from a marijuana pipe. (See "Michael Phelps: Eight Golds in Beijing")
Cowan did not specify what charge was being considered and declined to discuss details of the investigation. (See "Top 10 Athlete Drug Busts".)
The photograph was first shown in a British tabloid Sunday. News of the World said the picture was taken during a November house party while Phelps was visiting the University of South Carolina. (Top 10 Olympic Moments: Phelps' Photo Finish.)
Phelps and his advisers did not dispute the photo's authenticity. He has issued a public apology.
Meanwhile, the International Olympic Committee expressed confidence Monday that Michael Phelps will learn from his "inappropriate behavior" and continue to serve as a role model after a British newspaper published a photo of him inhaling from a marijuana pipe.
Phelps, who won a record eight gold medals at last year's Beijing Olympics, apologized and acknowledged "regrettable behavior" after the picture was published Sunday by the tabloid News of the World. "Michael Phelps is a great Olympic champion," the IOC said in a statement e-mailed to The Associated Press on Monday. "He apologized for his inappropriate behavior. We have no reason to doubt his sincerity and his commitment to continue to act as a role model." (Read "The Grass-Roots Marijuana Wars".)
During the Beijing Olympics, IOC president Jacques Rogge called Phelps "the icon of the games."
Marijuana is viewed differently from performance-enhancing drugs under World Anti-Doping Agency rules. An athlete is subject to WADA sanctions only for a positive test that occurs during competition periods.
The News of the World said the picture was taken during a November house party while Phelps was visiting the University of South Carolina.
The party occurred nearly three months after the Olympics while Phelps was taking a long break from training, and his actions should have no impact on the medals he won in Beijing.
He has never tested positive for banned substances and the case is unlikely to fall under any doping rules.