Quarterback Colin Kaepernick, who remains unemployed after a 2016 season in which he began the movement of players protesting during the national anthem, has filed a grievance accusing NFL teams of colluding to keep him out of the league, his legal representatives said.

Kaepernick retained Los Angeles-based attorney Mark Geragos to pursue the collusion claim and, according to a person with knowledge of the filing, it will be Kaepernick’s outside legal representation and not the NFL Players Association primarily in charge of preparing and presenting his case.

Geragos’ firm confirmed the grievance, saying it filed “only after pursuing every possible avenue with all NFL teams and their executives.”

In a statement, the law firm’ also said: “If the NFL . . . is to remain a meritocracy, then principled and peaceful political protest – which the owners themselves made great theater imitating weeks ago – should not be punished and athletes should not be denied employment based on partisan political provocation by the Executive Branch of our government. . . . Protecting all athletes from such collusive conduct is what compelled Mr. Kaepernick to file his grievance.”

The collective bargaining agreement between the league and the players’ union prohibits teams from conspiring to make decisions about signing a player. But the CBA also says the mere fact that a player is unsigned and evidence about the player’s qualifications to be on an NFL roster do not constitute proof of collusion.

For that reason, such cases are difficult to prove, according to legal experts.

“There has to be some evidence of an agreement between multiple teams not to sign a player,” said Gabriel Feldman, the director of the sports law program at Tulane University. “Disagreement over personnel decisions, as obvious as it may seem to someone looking at this, does not provide evidence of collusion. There has to be some evidence of an explicit or implied agreement. There has to be proof of a conspiracy.”

Kaepernick opted out of his contract with the San Francisco 49ers following last season, making him a free agent eligible to sign with any team. The 49ers have said they would have released Kaepernick rather than retaining him under the terms of that deal. He has remained out of work, being passed over by other teams in favor of other quarterbacks. The Seattle Seahawks and Baltimore Ravens considered signing Kaepernick but decided against doing so.

More recently, the Tennessee Titans signed Brandon Weeden to provide depth behind backup Matt Cassel when their starting quarterback, Marcus Mariota, was hurt. That signing seemed particularly inflammatory to Kaepernick supporters who cited Kaepernick’s superior career accomplishments. Kaepernick has led the 49ers to a Super Bowl and two NFC championship games and he threw 16 touchdown passes with four interceptions for them last season.

The NFLPA issued a written statement late Sunday saying it learned of Kaepernick’s grievance through media reports and that it had learned the league previously was informed of Kaepernick’s intention to file the grievance.

“Our union has a duty to assist Mr. Kaepernick as we do all players and we will support him,” the NFLPA’s written statement said, adding that it had been in regular contact with Kaepernick’s representatives over the past year about his options and planned to schedule a call for this week with his advisers.

Kaepernick refused to stand for the national anthem before games last season to protest, he said, racial inequality and police mistreatment of African Americans in the United States. Those protests were taken up by other players and the controversy over them has been amplified this season even with Kaepernick out of the league.

President Donald Trump called on NFL owners to “fire” players who protested during the anthem, referring to such a player as a “son of a b—-.” Vice President Mike Pence walked out of a game last week between the 49ers and Colts in Indianapolis, citing players’ protests. Trump indicated that he had orchestrated that plan.

Under pressure from the White House, NFL owners are scheduled to meet Tuesday and Wednesday in New York and might seek the NFLPA’s support of a measure for players to stand for the anthem, according to multiple people familiar with the sport’s inner workings, while also pledging league support for players’ community activism efforts.

Some media members have contended since the offseason that Kaepernick was being blackballed by NFL teams based on his political stance. NFL Commissioner Roger Goodell and owners were asked about that contention on a number of occasions and denied that teams were acting in concert on Kaepernick because of his protests.

“Each team makes individual decisions on how they can improve their team,” Goodell at conclusion of NFL owners’ meeting in May in Chicago. “If they see an opportunity to improve their team, they do it. They evaluate players. They evaluate systems and coaches. They all make those individual decisions to try and improve their team.”

Miami Dolphins owner Stephen Ross told reporters in July, according to the Palm Beach Post: “I would sure hope not. I know a lot’s been written about it, but you know owners and coaches – they’ll do anything it takes to win. If they think he can help them win, I’m sure – I would hope they would sign him.”

The plan for Kaepernick to pursue a grievance under the CBA was first reported by Bleacher Report.

“It may seem obvious to Colin Kaepernick,” Feldman said in a phone interview Sunday. “It may seem obvious to someone on the outside looking at this. But collusion requires an agreement [between teams]. Individual team decisions are not challengeable under the anti-collusion provision. An arbitrator is not going to second-guess an individual team’s personnel decision.”

If such evidence of collusion by NFL teams against Kaepernick exists, it has yet to revealed.

“We don’t know,” Feldman said. “Obviously everybody is talking about the baseball collusion cases from the 1980s, where there was a smoking gun. There were notes. There was strong evidence. There may be evidence here of collusion. We just don’t know.”

The NFL declined to comment Sunday through a spokesman.

“No Club, its employees or agents shall enter into any agreement, express or implied, with the NFL or any other Club, its employees or agents to restrict or limit individual Club decision-making,” the CBA says, adding that applies to “whether to negotiate or not to negotiate with any player” and “whether to offer or not to offer a Player Contract to any player,” among other things.

The CBA also says: “The failure by a Club or Clubs to negotiate, to submit Offer Sheets, or to sign contracts with Restricted Free Agents or Transition Players, or to negotiate, make offers, or sign contracts for the playing services of such players or Unrestricted Free Agents, shall not, by itself or in combination only with evidence about the playing skills of the player(s) not receiving any such offer or contract, satisfy the burden of proof set forth . . . above.”