ESPN has pulled out of a joint project with PBS' Frontline that is investigating head injuries in the NFL.

Frontline had been working for 15 months on the project with ESPN and is scheduled for release in October.

Sources say ESPN withdrew after coming under pressure from the NFL.

“At no time did we formally or informally ask them to divorce themselves from the project,” Greg Aiello, a spokesman for the NFL, told the Times. “We know the movie was happening and the book was happening, and we respond to them as best we can. We deny that we pressured them.”

ESPN pays more than $1 billion per season to the NFL to televise Monday Night Football.

ESPN claims that its lack of editorial control led to the withdraw,