Michael Phelps will not face criminal charges after a photo of him apparently smoking marijuana from a pipe sparked an investigation in South Carolina.

Richland County Sheriff Leon Lott said Monday that he didn't have enough physical evidence to press charges against the 23-year-old swimmer after the November party near the University of South Carolina in Columbia.

"We had a photo and him saying he was sorry for inappropriate behavior," Lott said at a news conference. "He never said, 'I smoked marijuana.' We didn't have physical evidence."

Lott defended the investigation, calling Phelps "an American hero … but even with his star status, he is still obligated to obey the laws of our state."

In a statement Monday, Phelps said he would move forward in his training, "having put this whole thing behind me."

"I'm glad this matter is put to rest," Phelps' statement said. "But there are also some important lessons that I've learned. For me, it's all about recognizing that I used bad judgment and it's a mistake I won't make again. For young people especially — be careful about the decisions you make. One bad decision can really hurt you and the people you care about."

The sheriff said the investigation began Feb. 2. Two days later, officers found the marijuana pipe thought to be used in the photo. They found it in a car. "The bong never made it to eBay," Lott said, referring to newspaper reports that the pipe had been posted on the online auction site.

Lott said the person who took the photo of Phelps at the party sold it for $100,000. He would not identify the photographer or say how he knew the amount.

The sheriff's office said eight people were arrested during the investigation. Seven have been charged with simple possession of marijuana, which carries a maximum penalty of 30 days in jail or a $575 fine. Another person was arrested for driving under suspension.

Columbia attorney Dick Harpootlian says he represents three clients charged with marijuana possession during the probe.

"I think it's a wise decision," Harpootlian said of the sheriff's announcement he would not charge Phelps. "I hope that the attitude towards Michael Phelps will carry over to my clients. Everybody needs to move on with their lives."

He praised Lott's commitment to law enforcement but added, "I think the process they went through, the two weeks of investigating all this, was not appropriate, but you're going to stumble occasionally. … We really think Leon Lott does a great job."

USA Swimming suspended Phelps from competing for three months. Kellogg Co. was the only sponsor to drop Phelps during the fallout.