The girlfriend, who called herself Lennay Kekua and claimed to be a Stanford graduate, was merely an online persona who "ingratiated herself with Manti and then conspired with others to lead him to believe she had tragically died of leukemia," university spokesman Dennis Brown said in a statement.
Notre Dame Athletic Director Jack Swarbrick said the university learned of the hoax from Te'o on December 26. He answered questions forthrightly and private investigators uncovered several things that pointed to Te'o being a victim in the case.
"This was a very elaborate, very sophisticated hoax perpetrated for reasons we can't fully understand but had a certain cruelty at its core, based on the exchanges that we were able to see between some of the people who perpetrated it," Swarbrick told a news conference.
Notre Dame's statements came after the website Deadspin.com published a long expose under the headline "Blarney," alleging that Kekua was a hoax dreamed up by a friend of Te'o's.
"Manti Te'o did lose his grandmother this past fall. Annette Santiago died on September 11, 2012, at the age of 72, according to Social Security Administration records in Nexis," the website said.
"But there is no SSA record there of the death of Lennay Marie Kekua, that day or any other. Her passing, recounted so many times in the national media, produces no obituary or funeral announcement in Nexis, and no mention in the Stanford student newspaper."
Deadspin said photographs identified as Kekua and shown in online tributes and on TV news reports belonged to a living 22-year-old California woman of a different name who is not a Stanford graduate, has never had leukemia and has not met Te'o.
Te'o, an All-American linebacker and finalist for the Heisman Trophy, college football's top individual honor, acknowledged in a statement carried by ESPN.com and the Honolulu Star-Advertiser, that he had never met Kekua in person.
But he said Wednesday he had developed an emotional relationship with her and "maintained what I thought to be an authentic relationship by communicating frequently online and on the phone," according to the statement.
"To realize that I was the victim of what was apparently someone's sick joke and constant lies was, and is, painful and humiliating," Te'o said.
On September 15, Notre Dame upset Michigan State 20-3 in a lopsided game where Te'o racked up 12 tackles - a considerable number.
It was a remarkable performance by the senior. But Te'o told his coach his grandmother and girlfriend had died just a few days before the game. The coach told reporters and Te'o's excellence became even more celebrated by the media.
Notre Dame continued to win and was preparing to meet Alabama in the national collegiate championship game on January 7 when Te'o told the university that he might be a hoax victim.
"The thing I am most sad about is that the single most trusting human being I have ever met will never be able to trust in the same way again in his life," Swarbrick said of Te'o. "That is an incredible tragedy."
The private investigators turned their final report over to the university on January 4. That report will not be made public, Swarbrick said.
Notre Dame lost to Alabama 42-14 three days later. Te'o is expected to be a first-round pick in the upcoming NFL draft.
(Reporting by James B. Kelleher and David Bailey; Editing by Cynthia Johnston, Philip Barbara and Lisa Shumaker)