HOUSTON — After a report by Al Jazeera in December linked Peyton Manning, the Philadelphia Phillies player Ryan Howard and other top athletes to performance-enhancing drugs, United States antidoping officials set out to investigate those claims with the help of Major League Baseball and the N.F.L.

Among the many leads being pursued, according to an antidoping official with direct knowledge of the case, is one involving a nurse practitioner in Houston who had committed suicide two months before Al Jazeera’s documentary was broadcast. The official requested anonymity because the investigation is confidential.

The United States Anti-Doping Agency has begun looking into the relationship between the nurse practitioner, Karen Lopez-Bartlett, and her fiancé, Charles Sly, the pharmacist at the center of the report, who spoke in detail about providing performance-enhancing drugs to several professional athletes. Ms. Lopez-Bartlett’s possible role highlights the unusual nature of the case.

Ms. Lopez-Bartlett ran a holistic medicine clinic in a building that also houses a compounding pharmacy, where technicians mix ingredients to create personalized medicines to treat conditions including pain, impotency and fatigue. Investigators from the antidoping agency are trying to determine if her prescriptions ended up in the hands of Mr. Sly and any athletes and, if so, whether the medicines contained banned substances.