University spokesman Dennis Brown said in a statement the Heisman Trophy runner-up was the victim of "what appears to be a hoax in which someone using the fictitious name Lennay Kekua apparently ingratiated herself with Manti and then conspired with others to lead him to believe she had tragically died of leukemia."
Brown said Notre Dame learned of the apparent hoax Dec. 26, 2012, almost two weeks before Alabama beat Notre Dame 42-14 in the NCAA Bowl Championship Series Jan. 7.
Te'o, 21, released a separate statement saying he was the target of "what was apparently someone's sick joke and constant lies."
He said he was duped into having a long-term "emotional relationship" with an Internet impostor -- a deception he called "painful and humiliating."
"To think that I shared ... my happiness about my relationship and details that I thought to be true about her just makes me sick. I hope that people can understand how trying and confusing this whole experience has been," his statement said.
The syndicated TV show "Inside Edition" said Thursday it has identified the woman whose image was used in the hoax as Diane O'Meara, a 23-year-old marketing professional from Los Angeles, and a former classmate of a friend of Te'o.
The Deadspin sports website, which broke the story about the apparent hoax Wednesday, raised questions about whether Te'o was duped or whether he somehow perpetrated the fictitious story of having a girlfriend who died in September.
Although both Te'o and Notre Dame knew about the fraud before the BCS title game, neither corrected the record until the Deadspin article was published.
Notre Dame said it had hired a private investigator who produced a final report Jan. 4, and the university shared the findings with the Te'o family Jan. 5.
Notre Dame Athletic Director Jack Swarbrick told reporters Wednesday night he believed the Te'o family had planned to come clean with the hoax story next week, but Deadspin beat them to the punch.
Swarbrick said neither Notre Dame nor Te'o tried to conceal the story. But Notre Dame acknowledged Wednesday it persuaded Te'o to wait and set the record straight after the title game.
"We encouraged him to try to focus forward and focus on the game," Swarbrick said.
Swarbrick said the Notre Dame investigation found the motive for creating a fake persona to trick Te'o had simply been the fun of it.
Online "chatter" among the alleged perpetrators suggested "the joy they were taking" in fooling Te'o, Swarbrick said.
The pranksters even made sure Te'o sent white roses in her honor and told him when they would close her casket, the Chicago Tribune quoted Swarbrick as saying.
"There was a place to send flowers," Swarbrick said. "There was no detail of the hoax left undone."
He likened the ruse to the 2010 film "Catfish" in which a woman built a fake Facebook persona with another woman's photo, and then duped another person into having a romantic online relationship.
"Nothing about what I have learned has shaken my faith in Manti Te'o one iota," Swarbrick said.
At the same time, the South Bend (Ind.) Tribune said Te'o and his family were unclear about the truth because they said Te'o met Kekua during their courtship.
Te'o described her to ESPN in October 2012 as the most beautiful person he had ever met. His father told the Tribune the same month Kekua had traveled to Hawaii, Te'o's home state, "every once in a while ... so he would meet with her there."