Waller's foundation held a fundraiser in the Henderson neighborhood of Las Vegas to help fight youth addiction, yet numerous players were seen on video without masks while interacting with guests during the indoor event at DragonRidge Country Club.
ESPN notes quarterbacks Derek Carr and Nathan Peterman were present, as were Jason Witten, wideout Zay Jones and cornerback Nevin Lawson.
Players are banned from activities that violate state and local COVID-19 safety regulations by an NFL and NFLPA agreement.
The Raiders have already received a $250,000 fine from the league and head coach Jon Gruden has been fined $100,000 for not properly wearing masks during a Week 2 matchup against the New Orleans Saints.
ESPN's Adam Schefter reported soon after that the NFL was investigating the team for allowing unauthorized locker room access following Vegas' victory over the Saints.
The Darren Waller Foundation event would mark the third known instance where members of the Raiders organization violated rules meant to keep players and staffers safer from the coronavirus while helping ensure the season can run as smooth as possible during the pandemic.
Speaking to Vincent Bonsignore and Michael Scott Davidson of the Las Vegas Review-Journal, Raiders governor Mark Davis expressed his disappointment in seeing players flout the guidelines.
"We obviously take responsibility for this," Davis said. "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 organization takes it very seriously."
The Raiders are scheduled to host the Buffalo Bills at Allegiant Stadium on Sunday afternoon.