An NBC Sports spokesman confirmed that Mr. Costas had been told that he had “crossed the line” but said it was not because he had discussed the topic of brain injuries and football. Instead, the spokesman said, it was because Mr. Costas had agreed after the University of Maryland symposium that he would no longer discuss the topic in interviews without prior approval from NBC Sports.

He had not sought approval for the CNN interview, the spokesman, Greg Hughes, said on Monday.

Shortly after the CNN appearance, Mr. Costas said, he was told he would not be part of NBC’s coverage of Super Bowl LII in February 2018. It was a sudden breakup between NBC and Mr. Costas, its longtime host of the Olympics and of “Football Night in America,” the network’s highly rated Sunday N.F.L. pregame show. Last year, he described the separation in public as “mutually agreeable.”

The parting was the culmination of Mr. Costas’s wavering commitment over many years to covering the N.F.L. He stepped away from NBC’s coverage in the 1990s, later saying the decision was because he “had ambivalent feelings about football.” Yet he was a host of HBO’s “Inside the N.F.L.” in the 2000s and then rejoined NBC’s coverage of the sport in the 2006-7 season, when the network started a $600 million-a-year deal with the league for Sunday night games. He continued hosting that program, “Sunday Night Football,” for the next decade, as awareness about the long-term effects of head injuries continued to rise.

During that time, Mr. Costas used the top-rated show’s platform to provide commentary on everything from guns to politics. In a halftime commentary in 2012, Mr. Costas called for “enlightened legislation and controls” on guns a day after a Kansas City Chiefs player shot and killed his girlfriend and then himself.

The network never received criticism or complaints from the N.F.L. about Mr. Costas’s coverage or commentary, including his remarks at the 2017 symposium, Mr. Hughes said.

By the time Mr. Costas appeared at that event, he had already taken a back seat in NBC’s coverage. He hosted the first “Football Night in America” of the 2017-18 football season but then handed off the remaining shows to Mike Tirico, who had moved over from ESPN to become NBC’s face of major sports events. When NBC eventually announced that Mr. Costas would not participate in the Super Bowl broadcast, he said that it would not have been fair to those who had been there the entire season.

“It wouldn’t be right for me to parachute in and do the Super Bowl,” he told The Associated Press weeks before the event. NBC and Mr. Costas officially parted ways last fall, when they agreed to end his contract early.