The NFL has decided to play hard ball with James Harrison, Clay Matthews, Julius Peppers and Mike Neal.

In a letter that was written to the NFLPA on Monday, and obtained by CBSSports.com, the league announced that each player will be suspended if they don't cooperate with the NFL's investigation into allegations that all four players received performance-enhancing drugs (PED) from the Guyer Institute in Indiana.

The allegations were made in December in a report that was released by Al Jazeeera America. Peyton Manning was also named in the report, but he has since been cleared. Much of the report was based on the testimony of Charlie Sly, who has since recanted everything he said.

The NFL has been attempting to interview Harrison, Matthews, Peppers and Neal for the past month, but nothing has gone down yet because the NFLPA has asked the players not to participate, according to the NFL's letter, which was written by Adolpho Birch, the NFL's senior vice president of labor policy and government affairs.

Back in June, the NFL had said that it planned to interview each player on the first day of training camp, something that still hasn't happened even though most teams are now three weeks into camp. The league has clearly grown frustrated with the NFLPA, which is probably why the players are now being threatened with a suspension.

The players have until Aug. 25 to do an interview with the league, and if that doesn't happen, they'll be suspended indefinitely, according to the letter.

"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," the letter states. "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."

The four players did turn in sworn affidavits in July, but the NFL rejected those for several reasons, including the fact that the league believes that Neal lied in his affidavit.

"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," the letter states.

Neal is currently a free agent.

Any potential suspension would hit the Packers the hardest since Matthew and Peppers both play for Green Bay. Harrison is going into his 13th year with the Steelers and 14th year overall.

The Steelers linebacker tried to fix the situation in June when he agree to be interviewed, but only under certain conditions. For instance, Harrison said the interview had to be done at his house on the date of his choosing, and Roger Goodell would have to attend.

James Harrison is one of four players facing an indefinite NFL suspension. USATSI

Harrison's agent, Bill Parise, told CBS Sports NFL Insider Jason La Canfora that Harrison would be open to an interview.

Based on the NFL's timeline, each player has 10 days to set of their interview, or else they'll be suspended.

Anyway, you can read Birch's entire letter below.

"As you know, the league has been investigating a nationally televised report concerning potential violations of the collectively bargained Policy on Performance-Enhancing Substances. There can be no question that the league has a good faith basis for conducting this investigation; moreover, the league and NFLPA have a shared interest in ensuring that our jointly developed policy is not being violated.

Nevertheless, since the initiation of our investigation in January the league has made at least seven attempts to arrange interviews of Messrs. Harrison, Matthews, Neal and Peppers. On each occasion, the NFLPA has communicated the players' refusal to participate. 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. Rather than eliminate the need for interviews, the players' plainly deficient statements simply underscore the importance of obtaining their full cooperation.

You were so advised on July 29, when we again wrote to offer the players another opportunity to participate in an interview, beginning with Mr. Neal. In that letter, you were expressly notified that continued noncooperation could result in discipline up to and including a suspension. In response, you provided a revised statement and letter which acknowledged Mr. Neal's prior steroid policy violation and that his previous "sworn statement" is in fact untrue. Most important, you advised that Mr. Neal had again refused to cooperate with our investigation by participating in an interview.

There is no dispute that players are obligated to cooperate with the league's investigation, as you have repeatedly acknowledged. This obligation includes not only the responsibility to submit to an interview but also the duty to provide meaningful responses to the questions posed. Nor is there a dispute that a failure to cooperate or an attempt to obstruct the investigation may result in discipline, including suspension from play, for conduct detrimental under Article 46 of the CBA and the NFL Player Contract.

We cannot accept your unilateral assertion that the cursory, untested statements you have submitted satisfy the players' obligation. Accordingly, the Commissioner has directed that Messrs. Harrison, Matthews, Neal and Peppers be given until Thursday, August 25 to provide interviews. 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. 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.

To avoid this outcome, please promptly contact my office to make arrangements for the interviews. As previously stated, we will make every effort to accommodate the NFLPA's availability, within the outlined time period."