The suspensions would begin Aug. 26, according to the letter.

“For those players whose interviews do not take place on or before that date, or who fail meaningfully to participate in or otherwise obstruct the interview, their actions will constitute conduct detrimental and they will be suspended, separate and apart from any possible future determination that they violated the steroid policy,” Birch wrote in the letter.

The union did not immediately respond to a request for comment.

The accusations against the Pittsburgh Steelers’ Harrison, the Green Bay Packers’ Matthews and Peppers, and Neal, an unsigned free agent, arose in an Al-Jazeera report last year.

The league has cleared retired quarterback Peyton Manning of wrongdoing. Manning and his wife were linked to shipments of human growth hormone by the Al-Jazeera report. Manning cooperated with league investigators, the NFL said previously.

The union has contended that Harrison, Matthews, Peppers and Neal are not required under the league’s rules to submit to interviews.

The league has made “at least seven attempts” to schedule interviews with the four players, according to Monday’s letter.

“On each occasion, the NFLPA has communicated the players’ refusal to participate,” Birch wrote in the letter. “Most recently, the NFLPA has attempted to prevent the interviews by submitting for each player a half-page statement, which you advised should be treated as a sworn statement given in a legal proceeding, and which you contend should fulfill the players’ acknowledged obligation to cooperate with the investigation. The statements, however, are wholly devoid of any detail, and we were quickly able to determine that Mr. Neal’s statement includes an assertion that is demonstrably false.”