Armstrong is under investigation by the US Food and Drugs Administration because there is a suspicion that he committed Federal offences. He rode for US Postal at the time, which was sponsored by the US Postal Service, an independent agency of the Federal Government
From Sports Illustrated:
"Because government sponsorship is involved, if evidence suggests that Armstrong was directing illegal doping activity, the inquiry could result in charges against him of conspiracy, wire fraud, money laundering, racketeering, drug trafficking and defrauding the U.S. government."
The sporting implications of the doping allegations are serious enough, but in Armstrong's case, it is much worse. The Feds are all over him because of potential criminal charges, and you don't get off with that just because you retired!