Brock had been arrested on suspicion of felony domestic violence in April, which led to his release by the San Francisco 49ers a day later. Charges were filed in June but dropped in August because of insufficient evidence.
The NFL investigation into Brock's arrest also found insufficient evidence that he violated the league's personal conduct policy, the source said.
The Seattle Seahawks traded Brock to the Vikings before the start of the season. He appeared in 11 games for the Vikings this season but missed the final two regular-season games with a foot injury