The league says Commissioner Roger Goodell "could not conclude" that Favre violated the league's personal conduct policy based on the evidence currently available.
The league's statement also says Goodell determined Favre was "not candid in several respects during the investigation resulting in a longer review and additional negative public attention for Favre, (Jenn) Sterger and the NFL." The league's investigation began in October and the fine was announced Wednesday.
Favre allegedly sent messages and photos to Sterger two years ago when they were both with the Jets. The allegations against the 41-year-old Minnesota Vikings quarterback surfaced on the website Deadspin.