Lindsay H. Jones

USA TODAY Sports

The NFL has issued its response to the NFL Players Association in the league’s quest to interview four players about allegations of the use of performance-enhancing drugs.

Adolpho Birch, the NFL’s senior vice president of labor affairs, on Thursday sent the union a letter stating that the union’s reluctance to make the players, Green Bay Packers linebackers Clay Matthews and Julius Peppers, Pittsburgh Steelers linebacker James Harrison and free agent linebacker Mike Neal, available for interviews was fundamentally at odds with the CBA.

In the letter, obtained by USA TODAY Sports, Birch says the allegations made in the documentary by Al Jazeera America in December warranted an investigation from the league, despite the fact that the source of the story, a former employee of an Indianapolis anti-aging clinic named Charlie Sly, immediately recanted.

NFLPA demands 'sufficient credible evidence' in league's James Harrison PED probe

The union, in a letter sent to the NFL this week, asked the league to provide any additional credible evidence that would warrant interviews with the players. Birch’s letter Thursday did not mention recently retired quarterback Peyton Manning, the most high-profile player in the Al Jazeera report. Because Manning is retired, he is no longer represented by the NFLPA.

“While we readily agree that such evidence is required to support the imposition of discipline, nothing in the CBA or the policy imposes such a requirement before possible violations of the policy may be investigated,” Birch wrote. “Obviously, the standard that you advocate — that the league cannot undertake an investigation unless and until it has established the facts and claims to be investigated — would simply ensure that there would be no investigations at all. For the same reason, we are under no obligation to disclose all evidence uncovered thus far as a condition to interviewing the players, which would clearly compromise the investigation.”

When reached for comment, NFLPA spokesman George Atallah told USA TODAY Sports the union had not yet received Birch's letter. Atallah said the union would be withholding comment until it had a chance to review it.

USA TODAY Sports reported last week that the NFL was planning to interview Neal on or before July 22, and the league planned to interview Matthews, Peppers and Harrison at their training camp facilities when they report to camp in late July.

Harrison responded that he would only cooperate if the interview were to take place before he reported for camp July 29 and that the interview take place at his home, with NFL Commissioner Roger Goodell present.

In his latest letter, Birch reiterated that the active players have an “obligation to cooperate with league investigations and may be disciplined for failing to do so.”

Follow Lindsay H. Jones on Twitter @bylindsayhjones.