After a week when Jon Gruden and the Las Vegas Raiders received heavy fines for violating sideline COVID-19 protocols, the team is now under an NFL investigation for another protocol-related violation - allowing unauthorized locker room access, league sources tell ESPN.

The alleged violation occurred after Monday night's win over the New Orleans Saints, when an unauthorized team employee entered the Raiders' locker room without the credentials to do so, according to sources.

The NFL will continue to enforce this limitation and will impose discipline for violations, according to a source. The league, with the help of state and federal committees as well as the NFLPA, has established strict thresholds regarding access -- meant to curb and prevent the spread of COVID-19.

This season, only 40 employees in each organization are allowed to be in the locker room, based on the NFL policy distributed to all teams. The list of employees allowed in the locker room, aside from the players, are the coaching staff, the athletic training staff, the equipment staff, the general manager, one member of team security and one member of the team's public relations/communications department.

The league believes there's evidence that a non-credentialed person eluded the security checkpoints to enter the Raiders' locker room Monday night, according to sources, and is now closely studying the incident and determining whether discipline is warranted.

The NFL already fined Gruden $100,000 and the Raiders $250,000 because Gruden was not consistently wearing his mask while on the sideline Monday night. Gruden, when asked after the game why he wasn't always wearing his mask, responded that he was "doing my best."

"I've had the virus," Gruden said. "I'm doing my best. I'm very sensitive about it. ... I'm calling plays. I just want to communicate in these situations, and if I get fined, I'll have to pay the fine. But I'm very sensitive about that and I apologize."