The NFL is still seeking an audience with the four linebackers it’s investigating for an alleged link to performance-enhancing drugs.

The league’s latest message to James Harrison, Clay Matthews, Mike Neal and Julius Peppers came with an ominous and onerous threat: Answer our questions in the next 10 days, or start your suspension.

NFL senior vice president of labor policy and league affairs Adolpho Birch sent a letter Monday to the NFL Players Association detailing a plan to suspend Harrison, Matthews, Peppers and Neal if they don’t speak with the league by Aug. 25.

Their punishment would then start the following day on an indefinite basis, to be ended at the discretion of Commissioner Roger Goodell once an interview has been completed.

Birch’s memo to the union was obtained by The Associated Press, according to a person with knowledge of the investigation who’s not authorized to speak publicly about it.

Harrison, a 14-year veteran, is a longtime leader for the Pittsburgh Steelers. Matthews and Peppers are key cogs in Green Bay’s defense, and Neal is currently a free agent who spent the past six seasons with the Packers. Harrison, Matthews and Peppers have been picked for the Pro Bowl a combined 20 times.

The NFL first notified the quartet on Jan. 11 about the investigation into a television report by Al-Jazeera featuring allegations made by Charlie Sly, who worked as an intern at an anti-aging clinic, about the use of banned performance-enhancing drugs by several athletes including the four linebackers. Sly later recanted his claims.

Birch, in his letter, said the league has made “at least seven attempts” to arrange the interviews.

Goodell’s power to punish has been a prickly issue between the NFL and the union in recent years, so the elevation of this dispute was not surprising since it revolves around the potential of player discipline and how it fits within the collective bargaining agreement.

The stance of the players has been steadfast in this case, refusing to be interviewed without being presented with what they’ve called credible evidence. Affidavits were sent by the NFLPA on behalf of the players to substitute for the interviews.