The NFL and the players’ union are looking for a new candidate to oversee a population study that would be a precursor to players being blood-tested for human growth hormone, union executive director DeMaurice Smith said Thursday.

That search must recommence after a mutually agreeable candidate withdrew from conducting the study, Smith said at an afternoon news conference outside the union’s offices in downtown Washington.

“We just recently found out . . . the league’s choice to run the population study that we had consented to recently withdrew and said that he wasn’t the right person to get this done,” Smith said. “I’m thrilled that the league has made a decision to move forward with the population study. I’m a little frustrated that their selection has now pulled out so that we have to again re-up and try to get this done.”

The league had no comment. But a person familiar with the league’s view of the situation, speaking on the condition of anonymity because of the sensitivity of the deliberations, said the league believes the union’s actions caused the scientist who had agreed to do the population study to withdraw. The candidate became convinced the study was scientifically unnecessary and thought the union’s actions indicated the study was more about politics than science, the person said, adding that other scientists have declined to be involved for similar reasons.

The Washington Redskins and Dallas Cowboys are among the defendants in the collusion lawsuit filed Wednesday by the NFL Players Association, even after the teams spent so much money on players during the sport’s season without a salary cap in 2010 that they incurred cap reductions by the league.

Union officials confirmed Thursday that all 32 NFL teams, including the Redskins and Cowboys, are defendants in the lawsuit filed in federal court in Minnesota. The complaint alleges that teams had a secret salary cap of $123 million per club during the uncapped year and improperly conspired to restrict players’ salaries.

“We look at what the facts are as we know it,” DeMaurice Smith, the union’s executive director, said at a news conference Thursday afternoon outside the NFLPA’s offices in downtown Washington. “The 32 teams are defendants in the action right now. If there’s evidence that is developed later on that would demonstrate that any one of those teams did not abide by the conspiracy, then my guess is those teams will make the appropriate assertions and we’ll see where we end up.”

Asked if he thinks the evidence indicates the Redskins and Cowboys did not participate in the alleged collusion, Smith said: “My view is the facts that we had justified and supported the complaint that we filed.”

The NFL Referees Association and the league are headed to federal mediation.

The Federal Mediation and Conciliation Service announced Thursday that the NFL and the referees’ association had accepted an offer for mediators to participate in their negotiation of a new labor agreement.

The current deal expires at the end of this month and the NFL has instructed its scouting department for officials to begin preparing for the possibility that replacement officials could be used during the upcoming season if a deal is not struck.

Both sides say they are intent on reaching a deal.

The Washington-based FMCS mediated the negotiations between the league and the players’ union last year before the players were locked out for 4 ¹ / ₂ months. The league and the players did not return to federal mediation when negotiations resumed, ultimately producing a 10-year labor agreement completed last summer. A court-appointed mediator in Minnesota oversaw settlement talks after players dissolved their union and filed an antitrust lawsuit against the owners.

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When he looked back on one of the most disappointing seasons of his 11-year career, Washington Redskins wide receiver Santana Moss knew something had to change.

A trimmer WR Santana Moss runs a route on the first day of organized team activities Monday (Photo by John McDonnell/The Washington Post)A year after recording a career-high 93 catches for 1,115 yards and six touchdowns, Moss in 2011 missed four games with a broken hand, lacked his usual effectiveness when he returned midway through the season and never got back on track. He ended the year with only 46 catches for 585 yards and four touchdowns -- his lowest output in nine seasons.