The NFL and the players union continue to bicker over an inflammatory -- and since denied -- Al-Jazeera America report linking several prominent players to PEDs.

The league filed the latest salvo, saying that the NFLPA should cooperate with the investigation in an email to the Pittsburgh Tribune-Review.

"The NFLPA and NFL are obligated and have a shared responsibility to look into allegations that could impact the integrity of competition on the field and the health of our players," NFL vice president of communications Brian McCarthy wrote in an email to the Tribune-Review. "We have been obtaining and reviewing numerous records, conducting multiple interviews and working with other entities. We have made no conclusions but the report merits a review, including interviews with the players named."

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The players named in the report, which were based on the claims of a supplement salesman named Charlie Sly, were since-retired Denver Broncos QB Peyton Manning, Pittsburgh Steelers linebacker James Harrison, Green Bay Packers linebackers Julius Peppers and Clay Matthews, and free-agent linebacker Mike Neal.

Harrison responded in his own way on Sunday, saying he'd meet with the league, but only on his terms.

The NFLPA, however, maintains that the league has no basis to seek interviews with the players named in the report at all.

Again, from the Trib-Review:

"The only thing we are saying to the league is, 'Show us what credible evidence you have, so that we can understand what the basis of your investigation is,' " Atallah said in a phone interview with the Tribune-Review. "Because if the dialogue exchanged in this media report from a guy who took back everything he said in a YouTube video, that's not enough."

The league's spokesman says the NFL has responded to the request for additional evidence, but Atallah contends there was "nothing substantive at all," included.