Tight end Fred Davis, who agreed to a deal to remain with the Redskins, is among the NFL players who have been unrestricted free agents this offseason. (Ricky A. Carioti/The Washington Post)

The players’ union has asked agents to provide any evidence they can to support suspicions that teams might be colluding to restrict players’ salaries on the free agent market this offseason.

DeMaurice Smith, the NFL Players Association’s executive director, wrote in a memo to agents that “we have heard reports of a concern that teams are working in concert to ‘peg’, ‘rig’ or ‘set’ market prices on player contracts. If you believe or have information that the teams have been colluding during this free agency period, you have a responsibility as an agent of the NFLPA to come forward and share that information with us.”

The league denied that collusion is taking place.

“Player signings in 2013 have been characterized by robust spending and intense competition,” Greg Aiello, the NFL’s senior vice president of communications, said in a written statement. “Anyone seeing collusion in this market is seeing ghosts.”

The collective bargaining agreement forbids teams from conspiring improperly to restrict salaries to players.

The union already has a collusion claim pending against the teams and the league. That claim accuses the teams of operating with a secret salary cap during the sport’s uncapped year in 2010.

The league denied that collusion took place in 2010 and contended that the union waived its right to make such a claim, both as part of the 2011 labor deal and in a separate agreement. A federal judge in Minnesota agreed with the league. U.S. District Court Judge David S. Doty twice ruled that the union’s collusion case could not proceed. But people familiar with the case have said in recent months that the union has not abandoned its claim and has continued to pursue it at both the district court and appellate court levels.

The league also prevailed in an arbitration case last year in which the Washington Redskins and Dallas Cowboys contested the salary cap reductions they were given by the NFL for the way in which they structured players’ contracts during the uncapped year. The league ruled that the two teams technically violated no salary cap rules but attempted to gain an improper competitive advantage. The NFL, with the union’s consent, reduced the Redskins’ salary cap by $36 million over two years and the Cowboys’ salary cap by $10 million over two years.

The two teams denied wrongdoing and challenged the penalties in arbitration under the CBA. But the arbitrator dismissed the case last year, ruling that the agreement between the league and union amounted to a rightful amending of the CBA. The Redskins continued to press the league this year for a lessening of their penalty. But NFL Commissioner Roger Goodell said at the annual league meeting last month in Phoenix that he’d told the Redskins the penalty would not be changed.

In Wednesday’s memo to the agents, a copy of which was obtained by The Washington Post, Smith wrote: “We have heard anecdotally that some teams are inaccurately reporting that they are facing salary cap restrictions on resigning veteran players. While this is a common allegation and teams are free to make their own determinations on signing players, we provide this information [about teams’ updated salary cap situations] to aid you in accurately evaluating each team’s actual salary cap room.”