The NFL has concerns about players placing sports bets and it is using its sportsbook partners to spy on customer activity and help identify guilty parties. The NFL Players Association (NFLPA) recently sent an email to agents explaining that the betting activity of players is subject to close observation by the sportsbooks themselves.

The email noted that sportsbooks, many of which the league has partnerships with, are “highly sensitive and very sophisticated at tracking” the locations of users. This is how the NFL discovered five players had recently breached the league’s rules.

The email closed by saying: “At no time should players open or use any mobile gambling app while at work.” NFL team staff and players cannot wager on any NFL-related betting markets. They are also not allowed to place any types of wagers when they are traveling with the team or at a team facility.

The NFL has partnerships with Caesars Entertainment, DraftKings, FanDuel, FOX Bet, BetMGM, PointsBet, and WynnBET.