The National Football League is demanding that The New York Times run a retraction after the paper published a report challenging the NFL’s research on concussions.

“On behalf of the NFL, a law firm sent a six-page letter to the Times’ lawyers demanding an immediate retraction of the story that was published online Thursday and on the front page of the Friday print edition,” the Wall Street Journal reports. “The letter claims the NFL gave the reporters evidence before publication that disproved the article’s central claims.”

WSJ adds: “The lawyer’s letter marks an escalation in the NFL’s attempts to undermine the story. Last week the league put out a 2,500-word, eight-point rebuttal on its website. Then the NFL ran a digital advertising campaign that included the unusual step of blanketing the Times’ online sports section Friday with banner ads highlighting the NFL’s safety initiatives. Some of the ads ran alongside the very article at the center of the feud.”

The Times is standing behind the article, which alleges that the NFL undercounted the number of reported concussions in its study of the issue and spotlights what the paper says are ties between the NFL and the tobacco industry.