A federal judge has declined to approve a proposed $765 million settlement between the NFL and thousands of former players suing the league over head injuries, ruling that the agreement may not go far enough to cover every player who may need aid.

The agreement, which was reached in August, set aside $675 million to compensate players for a specific list of injuries. League veterans who have been diagnosed with Lou Gehrig’s disease, for example, would be eligible for awards as high as $5 million. As much as $4 million would be awarded for a death involving traumatic brain injury, while players suffering from dementia would be be eligible for payments as high as $3 million.

More than 4,500 have signed on to the lawsuit, but according to U.S. District Judge Anita Brody, as many as 20,000 former players may one day be eligible for payment under the terms of the deal.

“I am primarily concerned that not all retired NFL football players who ultimately receive a qualifying diagnosis, or their related claimants, will be paid,” Brody wrote in her decision on Tuesday.

“Even if only 10 percent of retired NFL football players eventually receive a qualifying diagnosis,” she wrote, “it is difficult to see how the monetary award fund would have the funds available over its lifespan to pay all claimants at these significant award levels.”

Brody’s decision does not shatter the settlement. She ordered both sides to provide documentation showing that the deal will be large enough to cover all eligible parties.

The ruling marks a setback for the NFL in its bid to end a legal battle ignited by the discovery of the degenerative brain disease, chronic traumatic encephalopathy (CTE) in the brain of the late Pittsburgh Steelers center, Mike Webster.

In 2011, players brought suit against the NFL, claiming league officials knowingly concealed a link between traumatic brain injury and professional football. The backstory of the case is the focus of the FRONTLINE investigation, League of Denial: The NFL’s Concussion Crisis.

By settling the lawsuit the NFL avoided a potentially embarrassing discovery phase that was likely to draw new attention to the medical histories of league alumni. The agreement also ensured that league officials would not have to testify under oath about its handling of brain injuries. Players had initially sought $2 billion, but the suit ended up in court-ordered mediation after the judge told players’ attorneys their case was in “real danger of being dismissed,” ESPN’s Outside the Lines reported earlier this year.

“We respect Judge Brody’s request for additional information as a step towards preliminary approval,” a league spokesman said in a statement to FRONTLINE. “We will work with the plaintiffs’ attorneys to supply that information promptly to the court and special master. We are confident that the settlement is fair and adequate, and look forward to demonstrating that to the court.”

If Brody ultimately approves the agreement, players will have a 60-day window to decide on whether or not to opt in.

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