The NFL has threatened discipline, including suspension, for players refusing to cooperate with the league's investigation into steroid claims made by an Al-Jazeera America report.

Pittsburgh Steelers linebacker James Harrison, Green Bay Packers linebackers Clay Matthews and Julius Peppers and former Packers linebacker Mike Neal have until Aug. 25 to comply with the league's requests for interviews, according to an NFL letter sent to the NFL Players Association and obtained by ESPN.

Vice president of labor policy and league affairs Adolpho Birch wrote the league has a "good faith basis" for investigating potential violations of the NFL's drug policy, yet the league has made at least seven unsuccessful attempts to interview these players.

"For those players whose interviews do not take place on or before [Aug. 25], 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. "The suspension for each such player will begin on Friday, August 26 and will continue until he has fully participated in an interview with league investigators, after which the Commissioner will determine whether and when the suspension should be lifted."

A spokesman for the NFLPA said it doesn't have a comment at this time.

The NFLPA has previously contended there's no evidence to merit an investigation. Charlie Sly, the Indiana pharmacist who made the accusations to Al-Jazeera, has recanted his statements about the players, including Peyton Manning, who was central to the report and will not be punished by the league after cooperating with investigators.

The players have an obligation to "submit to an interview but also the duty to provide meaningful responses to the questions posed," Birch wrote.

The NFLPA issued statements to the league on behalf of the players, of which the NFL considers "wholly devoid of any detail." In his statement, Harrison denies the use of performance-enhancing drugs, and he recently told USA Today that the allegations made by Al-Jazeera were "flimsy."

Harrison has declined comment on the matter at training camp.

Matthews called the claims "bogus" at the start of training camp.

"It's kind of annoying that I have to continue to deal with this," Matthews said. "But the truth will come out, and everything I said when the allegations came out I still stand by."

Peppers called the allegations "nonsense."

The league first focused on interviewing Neal because he's a free agent, but the NFL's letter cites an assertion in Neal's NFLPA-approved statement as "demonstrably false."

"Rather than eliminate the need for interviews, the players' plainly deficient statements simply underscore the importance of obtaining their full cooperation," Birch wrote.

Players can contact the NFL office to schedule an interview, the letter states.

NFLPA assistant executive director of external affairs George Atallah told ESPN this summer that the players need more evidence from the league before addressing the reports further.

"They have proved to have a terrible track record when it comes to investigations," Atallah said of the NFL.

ESPN's Rob Demovsky contributed to this report.