There's no timetable to complete the investigation, but it's not expected before Denver plays Carolina in the Super Bowl on Feb. 7.
League spokesman Brian McCarthy says the inquiry, which began weeks ago, involves reviews of records, interviews and coordination with other agencies.
Al Jazeera reported last month that an intern at an Indianapolis anti-aging clinic was secretly recorded suggesting that Manning's wife received deliveries of HGH, which is banned by the league. Manning, then with the Colts, was rehabbing from shoulder surgeries.
The intern, Charles Sly, has since recanted his story.
Manning angrily denied using performance-enhancing substances and called the report "complete garbage."