Duerson sent text messages to his family before he shot himself specifically requesting that his brain be examined for damage, two people aware of the messages said. Another person close to Duerson, who spoke on the condition of anonymity, said that Duerson had commented to him in recent months that he might have C.T.E., an incurable disease linked to depression, impaired impulse control and cognitive decline. Members of Duerson’s family declined an interview request through a family friend.

Duerson was a four-time Pro Bowl safety, primarily for the Bears. He won Super Bowls with the Bears and the Giants and retired in 1993.

For the past several years, Duerson served on the six-person panel that considers retired players’ claims through the league’s disability plan and the 88 Plan, a fund founded in 2007 to help defray families’ costs of caring for players with dementia. So Duerson would have been familiar with the stories of hundreds of retirees with mental issues ranging from impaired short-term memory to outright dementia.

“You know he’s been sitting in the disability meetings and the applications, so I’m sure he’s seen a lot of disability applications that have to do with brain injury,” said Ben Lynch, a center for the 49ers from 1999 to 2002. “Having seen all those things come across in front of him, and for him to make the request about his brain, it’s something that must have been really on his mind. It’s unbelievable to me that this happened. The fact that he shot himself in chest, and not the head, it’s really eerie.”

Matt Birk, a center for the Baltimore Ravens, is one of 6 current N.F.L. players and 103 in all who have pledged to donate their brain to the Boston University center for analysis after their death. He said that Duerson’s requesting the same before shooting himself in a way punctuated the first era of the investigation.

“It’s almost now to the point that  not that it’s not tragic  but now it’s almost becoming common, some former players with some form of brain problems,” Birk said. “Is it something that I think about? Yeah, absolutely. There’s a little bit of, ‘Well, it’s not going to happen to me.’ ”