Las Vegas Raiders owner Mark Davis says the team has launched an internal investigation after a number of players attended a maskless charity event on Monday.
Quarterback Derek Carr was one of several Raiders to show up without masks at the fundraising event organised by tight end Darren Waller's foundation, violating NFL regulations and Nevada restrictions.
Video from The Darren Waller Foundation charity event, held at the DragonRidge Country Club in Henderson, Nevada, showed Carr and Waller were among a group of Raiders to attend the indoor function, that also included Jason Witten, Zay Jones, Nevin Lawson, Foster Moreau, Hunter Renfrow, Derek Carrier and Nathan Peterman.
They were mingling with guests in an effort to raise money for youth addiction but were not wearing masks.
Davis, who made a donation to the cause but did not attend, said that the team would look into the matter internally.
"We obviously take responsibility for this," he told the Las Vegas Review-Journal.
"You don't like seeing this. I don't know that it's actually been built into our memories that you have to wear a mask. Our organisation takes it very seriously."
The NFL and NFL Players Association have an agreement that bans players from activities that violate state and local COVID-19 safety regulations in an effort to prevent the spread of coronavirus.
The NFL already fined the Raiders $250,000 (£194,746) and head coach Jon Gruden $100,000 (£77,898) for not wearing their masks properly during a Week 2 win over the New Orleans Saints.
The team, according to ESPN, is also reportedly being investigated for allowing unauthorised locker room access following that Saints victory.
The country club was fined $2,000 (£1,558) for four state of Nevada violations, including people not wearing masks and more than 50 people at an event.