There is not sufficient evidence to determine a link between concussions and a brain disease that has been found in four dead hockey players, NHL commissioner Gary Bettman said. \"They\'re still looking at a very limited database,\" Bettman said. \"There\'s no control element because you have to look at everything that went on in a person\'s life before you can make a judgment as to what a brain may show when you open it up. \"There are no easy answers yet. I think it\'s unfortunate that people use tragedies to jump to conclusions that probably at this stage aren\'t supported.\" Bettman\'s comments follow a Boston University study of Derek Boogaard\'s brain that showed the former New York Rangers enforcer suffered from chronic traumatic encephalopathy (CTE), an ailment related to Alzheimer\'s disease. Boogaard died six months ago of an accidental overdose of the pain reliever drug oxycodone and alcohol. The 28-year-old Boogaard had more than 60 fights in his NHL career. The university\'s findings also revealed that Boogaard\'s disease was further along than in former Detroit Red Wing tough guy Bob Probert. Probert, who battled addiction problems, died at age 45 of heart failure. Former Ranger Reggie Fleming and ex-Buffalo Sabre Rick Martin were other hockey players who were found to have CTE. Flemming died at age 73 and Martin at 59. \"The findings released by Boston University regarding CTE found in Derek Boogaard\'s brain, and the forthcoming medical journal article, should be seriously considered by everyone associated with the game,\" NHL Players\' Association executive director Don Fehr said. \"It is certainly important information that we will be discussing with the players.\"