"I think the words were, 'You've crossed the line, '" Costas told ESPN in an article posted Sunday. "I thought that the movie would make an impact, and I thought this was a way not only for NBC to acknowledge it, but to get out in front of it".
Though he spoke out about concussions and brain injury in football throughout the 2010s, Costas made waves in 2017 at a symposium at the University of Maryland, when he said that "the reality is that this game destroys people's brains".
After WaPo published Costas' quotes, which also included "if I had an athletically gifted 12- or 13-year-old son, I would not let him play football", NBC released a statement saying, "Bob's opinions are his own, and they do not represent those of the NBC Sports Group".
Costas said he previously had agreed with NBC that Super Bowl LII in 2018 would be his last involvement in football, but that the network eventually reached out and told him he would be removed from their Super Bowl coverage.
Costas left NBC Sports formally in January after working for NBC Sports for four decades.
"I recall the phrase, 'It's a six-hour, day-long celebration of football, and you're not the right person to celebrate football, '" he told ESPN.
The breaking point in the relationship appears to be an essay Costas wrote for his once-usual address on "Football Night in America", the pre-game show before NBC's broadcast of "Sunday Night Football".