NFLPA's George Atallah questions evidence to back NFL's investigation into PED report

A six-month-old Al-Jazeera America report alleging several NFL players' use of performance-enhancing drugs has sparked the latest public spat between the NFL and NFLPA.

The NFL wants to interview Pittsburgh Steelerslinebacker James Harrison, retired quarterback Peyton Manning, Green Bay Packers linebackers Clay Matthews and Julius Peppers and free-agent linebacker Mike Neal, who were linked to the report.

But the NFL Players Association contends there's no evidence to merit an investigation.

Charlie Sly, the Indiana pharmacist who made the accusations to Al-Jazeera, has recanted his statements about the players, including Manning, who was central to the report.

NFLPA assistant executive director of external affairs George Atallah said the league has not provided additional information beyond the report to substantiate the claims. "We believe if there was something concrete, they would have shown us [already]," Atallah said Tuesday.

Citing "shared responsibility" by both sides to explore allegations that "could impact the integrity of competition," NFL spokesman Brian McCarthy said the league has reviewed records and conducted multiple interviews on the matter. "We have made no such conclusions but the report merits a review, including interviews with the players named," McCarthy said. The NFL hasn't shared documents with the NFLPA while it remains in the phase of gathering more information.

Atallah told NBC's "ProFootballTalk Live" that the NFL has notified the NFLPA that Major League Baseball and the U.S. Anti-Doping Agency are also looking into the matter. Free-agent catcher Taylor Teagarden, who was also linked to the report, is serving an 80-game suspension for violating baseball's drug policy.

The sticking point: While the NFLPA awaits facts, the NFL is trying to determine whether the allegations are indeed factual, which it believes is sufficient reasoning for a review. While the NFLPA has cited numerous times that Sly has changed his story, the league wants to know whether he was telling the truth originally or had motives to recant.

Atallah recognizes both sides probably define grounds for an investigation differently: "We happen to be right; they happen to be wrong," he said.

At the center of the probe is Harrison, who said on Instagram this week that he's willing to be interviewed on his terms: at his house, before training camp, with commissioner Roger Goodell present. Harrison has denied he took a drug called Delta-2, which Sly lists in the report. Harrison has pointed out that he's passed multiple offseason drug tests administered by the NFL. The league, which was prepared to conduct these interviews months ago, notified Harrison in a letter that it would like to interview him at training camp in Latrobe, Pennsylvania.

Atallah won't speculate but said he doubts Goodell will make his way into Harrison's house. Interviews like this typically include lawyers from both sides in a more formal setting.

Failure to cooperate with a league investigation could result in a suspension for the players.

The next step, according to Atallah, is making a recommendation to players on how to proceed, but the NFLPA needs more information before doing so.

After the contentious Deflategate investigation involving quarterback Tom Brady that's still floating in the U.S. Court of Appeals system nearly 18 months after allegations surfaced, the union won't take the league for their word in this case.

"They have proved to have a terrible track record when it comes to investigations," Atallah said.