The NFL Players Association is resisting the league's attempt to interview the active players named in a six-month-old Al-Jazeera America report on performance-enhancing drugs.
The players' union has formally requested evidence to support the NFL's investigation.
Retired quarterback Peyton Manning is likely to cooperate if requested, according to ESPN on Wednesday.
The players' union sent four letters to Adolpho Birch, the NFL's senior vice president of labor policy, in response to the league letter last week asking to interview four players named in the Al-Jazeera America report.
"On behalf of Mr. Harrison, we have repeatedly requested that the NFL inform him and the NFLPA whether the NFL possesses any credible evidence (e.g., verified documents or verified testimony of witnesses) that warrants an interview of Mr. Harrison regarding a potential violation of the Policy on Performance Enhancing Substances (PES Policy)," read the union's letter on behalf of Harrison, a copy of which USA Today Sports obtained. "The NFL's June 3rd response to our most recent written inquiry about this matter did not provide copies of any such credible evidence, nor did you inform us that the NFL possesses or is even aware of the existence of any such credible evidence. Thus, it appears that the entire basis for the NFL's investigation of Mr. Harrison consists of verbal remarks that appeared in a report broadcast by Al Jazeera and were subsequently recanted.
"Especially in a business where the mere mention of a player-employee's name can generate ratings for a broadcaster, the NFLPA and Mr. Harrison do not believe that unsupported, unsubstantiated verbal remarks provide 'sufficient credible evidence' to initiate an investigation of, and require an interview with, an employee.
"Mr. Harrison is a responsible employee who takes his professional obligations very seriously; therefore, if there is any credible evidence (beyond the verbal remarks in the report aired by Al Jazeera) that gives rise to a legitimate employer investigation of Mr. Harrison under the PES Policy, please share or describe that information so that he can reassess his obligations as an employee in light of any such information."
Birch informed the NFLPA that the league will meet with the three active players before the start of training camp.
Harrison wrote in an Instagram post Sunday night that a meeting with the NFL must take place at his house before training camp begins and only if commissioner Roger Goodell shows up for the interview.
Harrison posted a copy of the NFL's letter scheduling his interview for Thursday, July 28, the day before the Steelers open camp in Latrobe, Pa.
Manning was the highest-profile name linked to PEDs in the Al-Jazeera America documentary that aired in December. The report alleged that Manning was given a supply of human growth hormone in 2011 from an Indiana-based anti-aging clinic.
Manning, who announced his retirement following the Denver Broncos' victory in Super Bowl 50 in February, has no obligation to cooperate in the league's investigation as a retired player.
Manning has vehemently denied accusations he used human-growth hormone or PEDs during his recovery from neck surgery in 2011. Manning was not mentioned in the letter but the NFL investigation is still progressing, according to a USA Today story last Friday.
Sources told ESPN that Manning likely will cooperate and submit to an interview with the NFL.
The NFL confirmed to ESPN on Monday that it plans to interview Manning, in addition to the four active players.
According to ESPN, the players' union is concerned that Manning's cooperation could put the four active players in a difficult spot from a public relations standpoint and has had discussions with Manning on this matter.
The allegations in the Al-Jazeera report were made by Charlie Sly, a former anti-aging clinic intern who was secretly videotaped as part of the documentary. Sly has since recanted his story.
The Al-Jazeera report claimed that Harrison was among the players to receive shipments of a new performance-enhancing drug called Delta-2. Harrison denied the allegations.