Bryan D. Daly is a former federal prosecutor based in Los Angeles and partner at the firm Sheppard Mullin Richter and Hampton.
His hiring by Armstrong was first reported by The Daily Journal, a legal trade publication, and then by the New York Daily News.
Daly confirmed to the AP he had been hired by Armstrong in an e-mail late Wednesday night.
The investigation was spurred by accusations from Floyd Landis, a former teammate of Armstrong's on the U.S. Postal team, in a series of e-mails sent to cycling and doping officials this spring.
Armstrong has denied the allegations and questioned Landis' credibility. Landis was stripped of his 2006 Tour title for doping.
On Tuesday, three-time Tour de France winner Greg LeMond said he believes evidence against Armstrong in the federal probe will be
In an interview with The Denver Post, LeMond said he was happy when he received the subpoena to make a July 30 appearance in federal court in Los Angeles to testify in the investigation.
"The evidence will come from the investigation," LeMond told the newspaper, "and I believe it will be overwhelming."
LeMond also said he believes Landis was telling the truth in his descriptions of what he claimed was Armstrong's systemic doping program.