NHL Commissioner Gary Bettman’s reluctance to link hits to the head in hockey with a degenerative brain disease found in several deceased former players has reached Congress.

League attorneys have filed Bettman’s letter to Connecticut Sen. Richard Blumenthal with U.S. District Judge Susan Richard Nelson, who’s presiding over a class-action concussion lawsuit against the NHL. Bettman’s 24-page response to Blumenthal was dated Friday. He said research on the link between concussions and chronic traumatic encephalopathy “remains nascent.”

The Connecticut Democrat had written June 23 to Bettman about the league’s stance on CTE.

Blumenthal cited in his letter the discovery of CTE in the brains of six former players after their deaths in recent years. He is the ranking member of the Senate subcommittee on consumer protection, product safety, insurance and data security.

Bettman angered many former players last year with comments following the death of former NHL defenseman Steve Montador at age 35. Montador suffered from CTE, a diagnosis that can only be made after death. His family filed a lawsuit against the league that was eventually consolidated with the current class-action case.