Sources told the NFL Network the Cowboys cooperated fully with the league's investigation and the NFL discovered no wrongdoing.
Sources told the NFL Network the league began looking into possible tampering by the Cowboys last fall. The NFL's investigation stemmed from alleged comments made by Dallas coaches to Thomas before the teams faced off in Week 3 of the 2018 season.
Before the season started, Thomas informed the Seahawks that he wanted to leave the franchise if he didn't receive a contract extension. Seattle held its ground and never came to an agreement with Thomas, leading to trade discussions between the Cowboys and Seahawks.
Thomas suffered a season-ending broken leg in Week 4, wiping out the organizations' trade negotiations.
The three-time All-Pro safety becomes a free agent next month.
Seattle drafted Thomas in the first round (No. 14 overall) of the 2010 NFL Draft. He is a six-time Pro Bowl selection and won Super Bowl XLVIII with the Seahawks. He has 664 total tackles, 28 interceptions, 68 passes defensed and two defensive touchdowns in 125 career NFL games.