Travis Tygart, head of the US Anti-Doping Agency (USADA), has appealed for more information to help the ongoing investigation into Alberto Salazar.
A number of athletes, coaches and officials -- including Olympic champion Mo Farah -- have cooperated with USADA, with the most damning allegation to emerge into the public domain involving a testosterone gel.
Salazar, who has coached Farah in Oregon since 2011, has not denied testing the gel on his sons, but insists it was in order to avoid his athletes being sabotaged.
Tygart, the USADA chief executive who played a key role in uncovering Lance Armstrong's deception in cycling, accepts the investigation will need more information as it progresses, though.
"We certainly appreciate all those who have come forward and that includes Mo Farah, who came forward and testified under oath and answered all of our questions," Tygart told BBC Radio 4's Today Programme.
"It's still under way and part of our job is to continue to investigate it and we certainly want to ask anyone with relevant information to come forward and that will continue."
Tygart refused to be drawn on the possibility of Farah -- who has not been accused of any wrongdoing -- being implicated and backed new IAAF president Seb Coe's proposal to introduce an independent testing body within athletics.
"Look at the money flowing into track and field and the perception is terrible - it's as if the fox is guarding the hen house and it's terrible," he added.
"We've long said you can't promote and police your sport. That undermines the credibility even when you're attempting to do a good job.
"You're sort of damned if you do, damned if you don't and that's exactly why all anti-doping efforts should be turned over to an independent anti-doping organisation that has the will, determination and funding to actually win for clean athletes, not just window-dress.
"That seems to be what Seb is suggesting and it's a good idea."