Commissioner Roger Goodell sent a wide-ranging memo about concussions to NFL teams, saying the co-chairmen of the league's committee on brain injuries have resigned and that he is examining potential rule changes "to reduce head impacts."

A copy of the memo was obtained Tuesday by The Associated Press before the league issued a press release about its contents.

Goodell wrote that Dr. Ira Casson and Dr. David Viano, who have led the league committee on concussions since 2007, "have graciously offered to resign from those positions and to continue to assist the committee in its important work. We have accepted those resignations and are currently identifying their replacements."

Goodell said he wants to add new members "who will bring to the committee independent sources of expertise and experience in the field of head injuries."

Casson has come under attack recently from the NFL Players Association and members of Congress for criticizing independent and league-sponsored studies linking NFL careers with heightened risk for dementia and cognitive decline.

A message was left for Casson at his office. The NFLPA did not respond to a request for comment.

Goodell also said he met Monday with competition committee co-chair Rich McKay to review specific types of plays with an eye to evaluating possible rules changes "to reduce head impacts and related injuries in a game setting."

Among the other points addressed in the memo, which was addressed to chief executives, club presidents, general managers, head coaches, team physicians and head athletic trainers:

—Goodell wrote the NFL will "continue to invest in research designed to improve equipment safety, and we will urge players to make informed choices regarding the use of the most technologically advanced helmets."

—He noted that former NFL coach and TV analyst John Madden is leading a panel of coaches looking into reducing concussion risk outside of games. Among the possibilities under discussion: reducing offseason work, limiting helmet use and contact in practice, mini-camps and training camps.

—The league will hold a conference on concussions in Washington in June; team medical personnel "will be required to attend."

"Our game today is played with the understanding that medical decisions must always take priority over competitive interests. As a result, our sport has become safer for those who play it," Goodell wrote. "However, and particularly when player health and safety is involved, we always strive to do better."

Goodell publicly has been increasing his focus on concussions as the issue has drawn more attention this season, including at a congressional hearing in October.

Casson did not appear on Capitol Hill that day, and some House members complained when he did not testify. During the hearing, Rep. Linda Sanchez, a California Democrat, played a clip of a TV interview in which Casson denied evidence of a link between multiple head injuries in NFL players with brain disorders. Sanchez said that reminded her of tobacco companies denying a link between smoking and disease.

During interviews with 160 NFL players conducted by The Associated Press from Nov. 2-15, 30 replied they have hidden or played down the effects of a concussion. Half said they've had at least one concussion playing football.

High-profile players such as Brian Westbrook of the Philadelphia Eagles and running back Clinton Portis of the Washington Redskins have missed games in recent weeks because of head injuries. Last Sunday, the two starting quarterbacks from last season's Super Bowl left their teams' games after taking blows to the head: Ben Roethlisberger of the Pittsburgh Steelers and Kurt Warner of the Arizona Cardinals.

"I expect to advise you in the near future of additional steps that we have identified, both through our own work and in conjunction with outside experts and the NFLPA. These include new medical guidelines and research directed at the issue of long-term effects of concussions," Goodell wrote. "Our goal remains to make our game as safe as possible, to protect the health and safety of our players, and to set the best possible example for players at all levels and in all sports."