Floyd Landis has said the physiologist, now with RadioShack, helped him dope.
Allen Lim, an exercise physiologist for Team Radioshack, appeared on Wednesday before a federal grand jury charged with looking into doping in professional cycling.
Lim has ties to admitted dope cheat Floyd Landis, who lost his Tour de France crown after testing positive for doping. The disgraced Landis said Lim helped him cheat, claims Lim has denied.
The same panel spoke last week with Stephanie McIlvain, a long-time friend of Armstrong. Her attorney later said she told jurors she never heard Armstrong admit taking banned performance-enhancing substances.
Investigators have started interviewing witnesses and collecting evidence in their probe of whether or not Armstrong and other pro cyclists took banned substances. Testimony is kept secret and sessions are closed to reporters.
Armstrong has repeatedly denied all allegations that he ever took any sort of banned performance-enhancing substances, the latest coming from Landis to help touch off the current federal probe.