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 league and union agreed last year as part of their collective bargaining agreement that players would be blood-tested for HGH. They targeted the outset of last season to commence the testing. But the two sides first had to agree to testing procedures, and ongoing disagreements on those issues have kept the testing from beginning.

The union raised several concerns and asked for a population study to be conducted to determine the proper threshold for what would constitute a positive HGH test for athletes the size of NFL players.

NFL officials initially contended that such a study wasn’t necessary but later said they would consent to such a study if it would result in the testing beginning. League officials have continued to say the test is reliable and safe and they believe testing of players should begin immediately.

But Smith said Thursday the population study is not the lone remaining issue.

“The players believe in a clean game, but also players believe in a clean process,” Smith said. “And trading a bad process to get a clean game makes no sense. So what we have done from the beginning of this discussion is to insist on a process that’s fair, a process that is transparent. I’m thrilled that the league has come to accept our view about the necessity of a population study. But with the recent withdrawal of their doctor, we’re going to have to move forward and ensure that the right people are involved in the right process.

“… Our position is the population study is one issue as it relates to HGH. Are there other outstanding issues with respect to the overall drug policy that we would like to see? Absolutely…. Are there issues that we need to work out? Sure. But to say is that the only issue left? No,” Smith said.

On other issues:

●Smith reiterated that the league has given the union no evidence that New Orleans Saints players received cash payments for hits that injured opponents the past three seasons.

The union is aware that it also represents players who might have been the targets of bounties and would respond appropriately if given such evidence of a pay-to-injure scheme, Smith said.

“There’s been an allegation that these guys have done something wrong,” Smith said, “and as you guys know, we’ve made it abundantly clear that we have not seen one piece of evidence that would show that one of those players got paid to target a player and injure him and get him out of the game. . . . If there is evidence indeed that these players intentionally targeted someone for money, we have as much interest, if not more, in all of those facts than the National Football League. What we have insisted on is a process that provides the players with a chance to confront the truth.”

The union has filed two grievances challenging the league’s suspensions of four players in the bounty case, including a full-season suspension of Saints linebacker Jonathan Vilma.

The league maintains that NFL Commissioner Roger Goodell acted on strong evidence in imposing the suspensions. Goodell said Tuesday he expects that evidence of bounties will be released to the public at some point.

●Smith defended the union’s resistance to the owners’ vote Tuesday to require players to wear thigh and knee pads during games beginning in the 2013 season.

The league has maintained that the equipment requirements are a prudent safety measure. The union has contended that the changes must be collectively bargained and Smith said Thursday the sport has more pressing safety concerns.

“What we believe in is to engage in a disciplined process that improves player safety,” Smith said. “I understand the position that the league took and announced the other day on hip pads and thigh pads. It does seem somewhat ironic to me that there’s been discussions about hip pads and thigh pads and I frankly don’t remember one conversation about how we can develop a better mouthpiece or whether we should have uniform helmet standards in the National Football League. If the league wants to focus on hip pads and thigh pads right now, I think I understand why. . . .On a day when they want to talk about hip pads and thigh pads, I’m well aware of the discussions that we haven’t had.”