The Harris County Sheriff's Office said in a statement Thursday that Sanders was the subject of an investigation stemming from a June 15 complaint. Following an investigation, the Harris County District Attorney's Office presented the case to a grand jury, which determined Jan. 9 that no charges would be filed.
The statement did not include details about the case, but TMZ Sports reports that he was accused of sexual assault.
The sheriff's office said Sanders was among the "involved parties" interviewed as part of its investigation. Harris County includes the city of Houston, where Sanders has a home.
In a statement Thursday the Broncos said: "We are aware of the media report involving Emmanuel Sanders and are looking into it.''
League spokesman Brian McCarthy said the report was "under review."
Attempts to reach Sanders were unsuccessful.
Sanders just finished his fourth season with the Broncos after signing in 2014 as an unrestricted free agent. He was slowed for much of the season with an ankle injury he suffered in Week 6 and missed four games.
His 47 receptions marked his lowest total since 2012 and his 555 receiving yards were his fewest since 2011.
Sanders, who turns 31 in March, is one of several veteran Broncos players who face uncertain futures with the team. Broncos president of football operations/general manager John Elway has promised plenty of changes in the wake of the team's 5-11 finish.