An unnamed source who is said to have collaborated with the FDA in the past has told CyclingNews that Jeffrey Novitzky's investigation is probably not focused on individual dopers but, rather, on the suppliers of doping products to athletes. The source said that Novitzky's trip to Europe is an indication that the investigation is much bigger than just investigating Lance's doping and that Novitzky would not have travelled to Europe unless there was some compelling evidence that they are collecting.
This theory sounds very plausible to me. Remember, the investigation began with Rock Racing and looking into their use of, and presumably their supply of, doping products. Soon thereafter came Floyd Landis making allegations about systematic and widespread doping on Lance's team and in the peloton in general. Interestingly, the source speculates that the investigation may also be looking at the supply of not only approved drugs but experimental drugs. I've often wondered if the top athletes, such as Lance, may be able to obtain experimental drugs that are not even on the banned list or being tested for by WADA or other anti-doping agents. Here are some major quotes from CyclingNews: