In 2009, a leaked NFL research study seemed to mark a game-changer in the debate around football’s concussion crisis. Former players, the research found, suffered from memory-related diseases at a rate that was 19 times higher than the general population.

The study went to the heart of the question of prevalence — how many former players were suffering? And the fact that it was commissioned by the NFL only added to its significance.

“It was the people who the league hired to find out the answers to these questions, giving them the answers,” Alan Schwarz of The New York Times told FRONTLINE. Schwarz was given a copy of the study at a Manhattan steak house. “It was very deep throat,” he said. “You knew that this was going to be big.”

There was one problem: The league was backing away from the findings. In a message to Schwarz, NFL spokesman Greg Aiello said the study had shortcomings, noting that “there are thousands of retired players who do not have memory problems.”

The story sparked national attention. Within weeks, NFL Commissioner Roger Goodell was testifying on Capitol Hill. With the league’s concussion stance under intense public scrutiny, lawmakers were comparing the NFL to Big Tobacco.

Suddenly, the league was in damage control. It shook up its concussion committee, introduced new rules geared at player safety and donated $1 million to Boston University for brain research. As you’ll see in the following scene from League of Denial: Inside the NFL’s Concussion Crisis, the effort ended with a surprise acknowledgment from what Schwarz described as an “annoyed” Greg Aiello:

In the film, which premieres tonight beginning at 9 pm EST, FRONTLINE investigates the hidden story of the NFL’s response to head injuries. Through interviews with former players, scientists and other experts on the concussion issue, it examines what the NFL knew about the risks of such injuries, and when it knew it.

You can find out when League of Denial airs on your local PBS station here.