ESPN the Magazine senior writer Peter Keating examines possible explanations for why over 100 diagnosed concussions weren't included in head-injury studies conducted by the NFL. (3:02)

The NFL has demanded that the New York Times issue a retraction about a story last Thursday that said the league omitted concussion cases when it downplayed the effects of head injuries on players, according to a report.

Politico reported Tuesday that it had obtained a letter sent by the NFL to the Times saying the story was "false and defamatory."

"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," the NFL letter to the Times stated, according to Politico.

The Times sports editor Jason Stallman said Tuesday that the Times sees "no reason to retract anything."

The Times, citing confidential documents, reported on March 24 that more than 100 diagnosed concussions were left out of the studies conducted from 1996 through 2001. Star quarterbacks such as Steve Young and Troy Aikman were among the players who suffered concussions that were not counted, and zero concussions involving Dallas Cowboys players were documented in NFL research during that period.

The research, published in 13 peer-reviewed articles beginning in October 2003, was supposed to be based on a full account of all diagnosed concussions team doctors had reported between 1996 and 2001, but the missing concussions accounted for about 10 percent of the total number of cases, according to the Times.

"It should be an unmistakable red flag that a team does not report any concussions over multiple years," Dr. Robert Cantu, one of the peer reviewers who at the time criticized the committee's analyses, told The Times.

The NFL responded to the Times on Thursday in a statement, saying the report was "contradicted by clear facts that refute both the thesis of the story and each of its allegations."

"The studies that are the focus of the Times' story used data collected between 1996-2001," the NFL said. "They were necessarily preliminary and acknowledged that much more research was needed. Since that time, the NFL has been on the forefront of promoting and funding independent research on these complex issues."

The league also sent a memo to its teams regarding the report, saying the Times' story offered "very little that is new concerning this long-ago research and reflects little more than the pre-determined views of its authors."

The NFL on Friday ran ads in the Times -- both in print and online -- focusing on the changes made by the league and saying it wanted readers to "have all the information about all the work that we've done to improve the safety of the game," NFL spokesman Brian McCarthy told the Wall Street Journal.