Citing unidentified sources, ESPN's "Outside the Lines" said the Heisman Trophy winner signed items in exchange for a five-figure fee during his trip to Miami for the BCS championship game.
ESPN said sources told "Outside the Lines" that Manziel signed photographs, footballs, mini football helmets and other items at the request of autograph broker Drew Tieman.
Reached at his home Sunday night in Pembroke Pines, Fla., Tieman declined to make any comment or take any questions from The Associated Press.
ESPN reported that a source said James Garland, the NCAA's assistant director of enforcement, contacted Tieman and at least one person associated with the signings in June.
"We cannot comment on current, pending or potential investigations," NCAA spokeswoman Emily Potter told the AP in an email statement Sunday night.
Texas A&M also declined to comment.
"It is Texas A&M's long-standing practice not to respond to such questions concerning specific student-athletes," the school said in an email statement to the AP.
It's unclear what level of involvement the NCAA has at this point. When a player is believed to have broken rules, it's not uncommon for a school to declare that player ineligible, then ask the NCAA to investigate and reinstate the athlete's eligibility.
Nate Fitch, a friend of Manziel's, posted several tweets at that time saying he was with the Heisman winner in South Florida for the BCS title game. None of Fitch's tweets around the dates of that trip suggested anything about meeting with Tieman, or any memorabilia signings.
Tieman's Facebook page was taken down Sunday night, but cached images collected from the page included a photo of him and Notre Dame's Manti Te'o, which was purportedly taken Jan. 3, less than a week before the BCS game.