The National Football League is stepping up its aggressive campaign to counter a New York Times investigation last week that questioned the league’s concussion research and claimed it had ties to the tobacco industry. 
 
 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 letter claims the NFL gave the reporters evidence before publication that disproved the article’s central claims. 
 
 “By publishing the story, fully aware of the falsity of the underlying facts, the Times recklessly disregarded the truth and defamed the NFL,” according to the letter from attorney Brad S. Karp of Paul, Weiss, Rifkind, Wharton & Garrison LLP. 
 
 Jason Stallman, the Times’ sports editor, defended the publication’s reporting on the NFL’s ties to the tobacco industry and its assertion that the league’s concussion studies were deeply flawed. 
 
 “We see no reason to retract anything,” Mr. Stallman said in a statement. The Times also ran a point-by-point counter to the NFL’s objections online last week. 
 
 Politico Media earlier reported the NFL’s letter demanding a retraction from the Times. 
 
 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 story alleged that the NFL undercounted the number of reported concussions it used to study the issue, a point Mr. Stallman reiterated on Tuesday. The league has defended its use of the data and has said it was transparent about the data’s limitations. 
 
 “The fact that the studies relied on a data set that was not a complete count of all concussions in the NFL—the central factual assertion in the story—was repeatedly and expressly disclosed in the studies themselves and counting all concussions was not the purpose of the studies in any event,” according to the NFL’s letter. 
 
 The article also claimed there are connections between the NFL and the tobacco industry that included shared lawyers, consultants and lobbyists. A league lawyer told the Times that it had no affiliation with the tobacco industry, and the letter called such a claim a “knowingly false and incendiary charge.” 
 
 “The Times’’s intent was obvious,” the letter stated. “It sought to publicly associate the NFL with the tobacco industry and all that implies, in a ‘front-page’ story, no less—without a scintilla of evidence to support such a link.” 
 
 The NFL asked that the reporters and editors involved in the investigation preserve all of their notes and correspondence related to the story. 
 
 Write to Nick Niedzwiadek at nick.niedzwiadek@wsj.com ||||| The National Football League is demanding The New York Times retract an article published last week about the league's research into concussions among pro football players. 
 
 In a letter that was sent to the Times' legal counsel Monday and obtained by POLITICO, an attorney for the NFL, Brad S. Karp, called the story "false and defamatory" and issued a "demand that the story immediately be retracted." 
 
 In a line that seemed to threaten the possibility of further legal action, he wrote: "We also request that the Times's reporters and editors who worked on this story preserve their notes, correspondence, emails, recordings and work papers and all other electronic and hard copy documents generated or received in connection with their work." 
 
 In an exhaustive front-page story on March 25, journalists Alan Schwarz, Walt Bogdanich and Jacqueline Williams reported that the NFL's concussion research was flawed, likening the league's "handling of its health crisis to that of the tobacco industry, which was notorious for using questionable science to play down the dangers of cigarettes." 
 
 The NFL has been aggressively countering the Times' reporting. 
 
 Spokesman Joe Lockhart published a 2,500-word rebuttal on the league's website, arguing that "the paper published pages of false innuendo and sheer speculation based on a mere handful of anecdotal and cursory references, twisted and contorted out of context, from a smattering of documents out of millions found on the tobacco litigation website." 
 
 Additionally, the NFL ran ads on nytimes.com to promote the steps it says it has taken to advance player safety; at least one of the ads appeared in a banner placement directly above the concussions story. The NFL also paid to promote its rebuttal on Twitter. 
 
 Sports editor Jason Stallman said the Times sees "no reason to retract anything." 
 
 In a statement to POLITICO, Stallman said, "Our reporting showed that more than 100 such concussions — including some sustained by star players — were not included in the [NFL's] data set, resulting in inaccurate findings." He also noted that a co-owner of the New York Giants has direct ties to the tobacco industry and that "the N.F.L. and the tobacco industry shared lobbyists, lawyers and consultants." 
 
 The NFL attorney argued in his letter that the Times piece was based on a "grand total of five pieces of circumstantial evidence, none of which—taken together or individually—comes close to establishing any meaningful 'tie' that reasonably can form the basis of the Times's knowingly false and incendiary charge." 
 
 Asked whether the NFL would sue if its demand for a retraction is not met, Lockhart said, "We won't prejudge the reaction of The New York Times. We make a strong argument for a retraction, one we expect them to take seriously." 
 
 High-profile investigative pieces like this one tend to invite intense push-back. 
 
 Last year, a Times exposé on the treatment of New York nail salon employees brought angry protesters to the paper's headquarters on Eighth Avenue in Manhattan. And after its investigation into the workplace culture of Amazon, the technology company's spokesman, Jay Carney, posted a rebuttal on the publishing platform Medium, to which Times executive editor Dean Baquet responded in his own point-by-point Medium post.
– Last week, the New York Times published an article claiming the NFL's much-touted concussion research was wildly flawed while simultaneously linking the league's methods to those of the much-reviled tobacco industry. Now, Politico reports the NFL is demanding the Times immediately retract the story, which it calls "false and defamatory," and insinuating it may take legal action against the paper. The NFL claims it gave the Times evidence disproving the story before it ran, according to the Wall Street Journal. “By publishing the story, fully aware of the falsity of the underlying facts, the Times recklessly disregarded the truth and defamed the NFL,” a letter from an NFL attorney states. Monday's letter to the Times' legal team was just the most recent step in the NFL's campaign against the Times' article. The league ran a 2,500-word rebuttal on its website that accused of the Times of "false innuendo and sheer speculation." It also covered the Times' online sports section in ads touting its new player safety measures. However, it appears the Times is standing by its story, with the paper's sports editor saying it has "no reason to retract anything."