This is something I posted on another board that applies to PED users in a variety of sports:
It was the FBI that did the heavy lifting in the Armstrong investigation and then just handed over its files, which included all the sworn testimony, to USADA. Without various law enforcement agencies on both sides of the pond (eg. Operacion Puerto, Festina scandal, Balco, Angel Heredia, etc.), there's a long list of athletes in cycling, track & field and baseball who would have never been exposed as dopers. Neither the UCI, the IAAF, MLB nor any of the other sports governing bodies will ever be able to stop athletes from doping until they get the power to issue subpoenas and conduct search warrants, because most people are afraid to lie under oath.
One big difference between doping in cycling and doping in other sports is that in cycling it's a team effort, versus individual efforts in other sports, which means that cyclists always have at least a dozen potential witnesses who can rat them out. This means that Armstrong had a lot more people to worry about than Barry Bonds and Roger Clemens. Bonds only had Greg Anderson (he went to prison instead of testify) to worry about and Clemens only had Brian McNamee (he got discredited due to his criminal record) and Andy Pettitte (he pulled a Frankie Pentangelli) to worry about.