HENDERSON, Nev. -- Coach Jon Gruden said Las Vegas Raiders players "let their dauber down a little bit" when they broke COVID-19 protocol by appearing maskless at an indoor charity event Monday night. Quarterback Derek Carr, who attended the event, said the players "had a few moments where we slipped up, took the masks off so people could see our face."

However, both pivoted to the reason the players were at the event in the first place: It was a fundraiser for teammate Darren Waller's foundation to help at-risk youth overcome addiction. Waller himself is now three years sober.

"I hate that a few moments without our masks on led to a story about our team and all this kind of stuff, especially after the fines that were brought on a couple of weeks ago," Carr said Wednesday. "So, we felt terrible about that. We addressed it, we talked with Coach and we talked with our trainers about what really went down and all that kind of stuff."

News of the Raiders appearing at the event without masks surfaced one day before the Tennessee Titans reported positive COVID-19 tests from four players and five team personnel members.

Earlier this month, Gruden was fined $100,000 for not wearing his mask properly on the sideline during Las Vegas' win over the New Orleans Saints on Sept. 21. The Raiders were fined $250,000 and also are being investigated by the NFL for allegedly allowing an unauthorized team employee in the locker room after that game.

Meanwhile, the NFL has issued another strong warning to coaches who won't wear masks on the sideline during games, threatening suspensions and forfeiture of draft picks as punishment for failing to comply with COVID-19 game-day protocols, according to a league memo obtained Wednesday by ESPN's Adam Schefter.

"We all understand that this is a terrible, terrible virus," Gruden said. "And we have to do our part. We're all vulnerable. Unfortunately, it sounds like a few people got the virus. I just wish the best for everybody in Tennessee. And it does certainly get yourself awakened to the reality of, it can get anybody, anywhere, at any time."

Gruden, who has said he had the coronavirus in July, has made it a team mission to "crush the virus." During training camp, he had T-shirts made with that slogan along with the Raiders logo on them.

"I'll just say this: We've done a good job, we've done an excellent job," Gruden said. "Last night, it was addressed with our players. They walked in with their masks on, there was an event. Sometimes you go to a restaurant, take your mask off. They're aware of their mistake.

"But we've done an excellent job -- using our masks, taking proper care of each other and everybody. I'm really proud of our players. And by the way, that was a great cause no one's talked about. [Waller] raised over $300,000 for a great cause. I appreciate our players being there in support of him."

An NFL spokesperson said in an email to ESPN.com on Wednesday afternoon that the league was "looking into the matter," which violated Nevada state regulations for the pandemic and might have broken league regulations.

NFL and National Football League Players Association rules limit what activities players are allowed to engage in when away from team facilities. Players, it is stated, are not allowed to attend any event that "violates local and state restrictions."

The Raiders are slated to host the Buffalo Bills on Sunday. Bills cornerback Tre'Davious White said the issue was "definitely something that guys were mentioning in the locker room" but added that the team mostly was not concerned.

"It raised a lot of antennas. But I feel like if we continue to take the precautions and do all the things we're supposed to do and go in the places we're supposed to go and not be selfish with it, I think we'll be fine," White said.

Added Bills coach Sean McDermott, "What we try to do is control what we can control from our end. Got a lot of respect for Coach Gruden, and I'm sure they're on top of it over there."

Joining Carr at Waller's event were backup quarterback Nathan Peterman; tight ends Jason Witten, Foster Moreau and Derek Carrier; wide receivers Zay Jones and Hunter Renfrow; cornerback Nevin Lawson; and center Erik Magnuson.

The event was held indoors at the DragonRidge Country Club, which was fined $2,000 by the city of Henderson for four violations of the Nevada governor's COVID-19 emergency directives, including people not wearing masks and more than 50 people at the event. The club now has 30 days to pay the fine or dispute it.

Raiders owner Mark Davis told ESPN on Tuesday night, "Guys have to be more stringent in fighting the virus. It's still our toughest opponent."

Carr, who posed for pictures with a maskless guest who won his jersey in an auction, said he realized not wearing a mask to the event was a mistake.

"We signed waivers," Carr said. "Hand sanitizer, like crazy. ... We weren't perfect, but we weren't trying to be careless or reckless. But at the same time, [Waller] had an event for something that meant a lot to him and they raised so much money that you won't even begin to imagine how much money they raised to help other people that struggle with addiction, to get them in the right place and things like that. My hope and my prayer is that a few moments of us messing up, a few minutes here, where we were seen on camera, not in the private room, in the separate room for an hour, I hope that we don't lose what was really going on. Because Darren had a great idea. He had a great plan. He wants to help people.

"We should have kept the masks on, even if we are coming in and they're introducing us. ... I was just there trying to support Waller. That's what me and the guys were trying to do, but that's the end of that."

Information from the Associated Press was used in this report.