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While M.I.A. has offered no official comment on why she threw up a middle finger during Sunday's Super Bowl halftime show, a source close to the rapper told ABC News Radio that her actions were caused by "adrenaline and nerves,"  a result of getting "caught up in the moment," and "weren't an attempt to make any kind of statement."

Regardless, M.I.A. flipping the bird and uttering an expletive to an audience of millions has become one of the most talked about moments from Super Bowl XLVI. The gesture came during a performance of Madonna's new single, "Give Me All Your Luvin," on which M.I.A. raps. Viewers at home saw their TV screens blur for a second afterwards in a failed attempt to conceal the act.

"The NFL hired the talent and produced the halftime show," NBC spokesman Christopher McCloskey said. "Our system was late to obscure the inappropriate gesture and we apologize to our viewers."

The NFL blamed a failure in NBC's delay system for allowing the gesture to be seen. Spokesman Brian McCarthy said M.I.A. did not do anything similar during rehearsals and the league had no reason to believe she would pull something like that during the actual show.

"The obscene gesture in the performance was completely inappropriate, very disappointing, and we apologize to our fans," McCarthy said.

The British singer is best known for her 2007 hit "Paper Planes," a Grammy nominee for record of the year. Representatives for M.I.A. and Madonna did not immediately respond to ABCNews.com's requests for comment.

The backlash is picking up. Today, the Parents Television Council issued a statement asking for more than a "simple apology" from NBC and the NFL and slammed them for choosing "a lineup full of performers who have based their careers on shock, profanity and titillation."

"Instead of preventing indecent material, they enabled it," PTC president Tim Winter said. "M.I.A. used a middle finger shamelessly to bring controversial attention to herself, while effectively telling an audience filled with children, 'F- you.'"

So will M.I.A.'s finger flip result in a heavy FCC fine for NBC, like the $550,000 one the agency slapped CBS with after Janet Jackson's 2004 Super Bowl exposure? We asked Drexel University law professor Lisa McElroy.

"There is reasonable potential for an FCC fine," she said, citing the FCC's recent fines against broadcasters for airing fleeting expletives, like Nicole Richie and Bono's F-bomb drops at two separate award shows in 2003. After the Jackson incident in 2004, the FCC increased the fine for each indecency to $325,000, which means that M.I.A.'s combined expletive and middle finger could cost NBC as much as $650,000.

But, McElroy pointed out, the FCC's "fleeting expletives" issue is currently before the Supreme Court. They heard arguments in January and will decide the case by the end of June. "Given that the court might limit the FCC's ability to fine in situations like these," she said, "the FCC might want to be somewhat less assertive than it has been in other recent situations."

The Associated Press contributed to this report.