A lawyer representing Tyler Hamilton told ESPN.com on Monday he felt compelled to notify federal authorities about an incident involving his client and fellow cyclist Lance Armstrong.
Hamilton, who has accused the seven-time Tour de France winner of doping, and Armstrong had a conversation in an Aspen, Colo., restaurant Saturday night.
Lawyer Chris Manderson said Hamilton told him Armstrong made derisive comments, while the restaurant co-owner and friend of Armstrong told ESPN.com: "It was not a big confrontation."
Federal officials are now in their second year of investigating doping in cycling.
A Los Angeles-based grand jury is hearing evidence that could lead to charges of fraud, conspiracy and drug trafficking against Armstrong and his team.
"Lance Armstrong is a possible defendant in an investigation that's been widely reported and Tyler is a probable witness," Manderson told the website.
"When there's any contact, especially aggressive contact, we, as lawyers, have a duty to inform the authorities … What they will do with it, I don't know."
A spokesman for Armstrong declined comment when approached by ESPN.com.
Armstrong told Outside Magazine the incident was "certainly awkward for both of us" and "truly uneventful."