Shortly before the NFL announced that Arizona Cardinals defensive back Josh Shaw was suspended through at least the 2020 season, the league sent a memo to all teams reaffirming that any employee found gambling on football would be terminated, league sources told ESPN.

The league has outlined its stance in the past, but in light of Shaw's recent suspension for betting on NFL games on multiple occasions this season, it felt the need to remind all 32 teams again.

The Cardinals were unaware the league was investigating one of its players, according to sources, because Shaw spent only a brief period of time with the team this past summer before being placed on injured reserve.

Since then, Shaw has not been around the Cardinals' complex, and they have not maintained any regular contact with him. So when the league publicly announced Shaw's suspension Friday, it surprised the Cardinals.

The NFL's investigation into Shaw did not uncover evidence that indicated he used inside information or that the integrity of any game was compromised. The NFL also announced that Shaw's teammates, coaches and other players around the league were not aware of Shaw's gambling.

Betting on football is one of 10 restrictions on legal gambling that the NFL laid out in its gambling policy.

"All NFL Personnel are prohibited from placing, soliciting, or facilitating any bet, whether directly or indirectly through a third-party, on any NFL game, practice or other event," the policy states. "This includes betting on game outcome, statistics, score, performance of any individual participant, or any other kind of 'proposition bet' on which wagering is offered."

Shaw can petition for reinstatement on or after Feb. 15, 2021, if his potential appeal is denied.

Suspensions of NFL players for betting are rare. The highest-profile case was in 1963, when Paul Hornung of the Packers and Alex Karras of the Lions were banned for the entire season. Both were reinstated the following season, and Hornung was eventually inducted into the Hall of Fame.

In the most recent case, Baltimore Colts quarterback Art Schlichter was suspended in 1983 for betting on NFL games and other sporting events. He, too, was reinstated the following year.

The Cardinals signed Shaw on March 18 to a one-year contract worth $850,000. He has made more than $3 million in his career.

Shaw, 27, was drafted by the Bengals in the fourth round of the 2015 draft. After playing his first three seasons in Cincinnati, Shaw split time between the Chiefs and Buccaneers last season.

ESPN's Josh Weinfuss and The Associated Press contributed to this report.