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GREEN BAY - Threatened with suspension if they do not cooperate with an NFL investigation into allegations of performance-enhancing drug use, Green Bay Packers linebackers Clay Matthews and Julius Peppers will submit to the league’s interview request, USA TODAY Sports learned Thursday.

An Aug. 25 deadline to submit for in-person interviews was imposed in a letter NFL senior vice president of labor policy and league affairs Adolpho Birch addressed to the NFLPA. The letter was obtained by USA TODAY Sports on Monday.

Matthews and Peppers, along with former Packers linebacker Mike Neal and Pittsburgh Steelers linebacker James Harrison, faced suspension if they did not cooperate before the deadline.

"I’m just glad the process is moving forward," coach Mike McCarthy said after Thursday night's preseason game against the Oakland Raiders. "As already has been stated as an organization, we support Clay and Julius. We’re looking to get this resolved as soon as possible."

It is unknown when Matthews and Peppers will speak with NFL investigators. Neal has not yet agreed to an interview, a person with knowledge of the situation told USA TODAY Sports. Harrison said he would speak with league officials at 5 p.m. Aug. 29, four days after the deadline.

The NFL is looking into allegations of PED use made in an hour-long documentary from Al Jazeera America released last December. The report’s leading source, former anti-aging clinic intern Charlie Sly, implicated several NFL players, including Matthews, Peppers and Neal, in secret recordings. Sly subsequently recanted his accusations.

Matthews and Peppers emphatically denied the accusations after a Dec. 27 game at Arizona.

In an opening statement with media earlier this week, general manager Ted Thompson said there was no contingency plan to play without Matthews and Peppers this season.

“Before we get started, I’d like touch on a couple of things, specifically the media reports involving Clay and Julius,” Thompson said. “It’s kind of as simple as pie: We’re going to support our players, we’ve always supported our players, and we will continue to support our players. This is no different in this case.

“I will have nothing further to say on this matter. Everybody in the world’s going to be talking about it, but I’m not going to have anything to say about (it), the Packers will not have anything to say about it until it gets resolved. You can in the meantime direct your questions to the NFL or the NFLPA.”

Both Matthews and Peppers were inactive for the Packers' preseason game Thursday against the Oakland Raiders.

A letter from NFL Players Association counsel Heather McPhee to Birch – obtained Thursday by USA TODAY Sports – accuses the NFL of “public shaming” and asks whether the league has any credible evidence beyond the initial allegations made by Sly.

“Mr. Harrison and the NFLPA agree that this interview is occurring on a non-precedential basis, because of Mr. Harrison’s professional decision in this situation to respond to an employer who has failed in its obligations to him, and he reserves all of his rights regarding any future action the NFL may attempt to take in this matter,” the letter said.

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