The message said "Remember, remember the fifth of November." That reference suggested a link to Anonymous, a movement of cyber rebels which has appropriated much of the symbolism around Britain's upcoming Guy Fawkes Day, a holiday meant to mark the failed plot to blow up the country's parliament in 1605.
However, conclusively determining responsibility for such attacks can be nearly impossible.
Also defaced by "pyknic" Sunday was a Lady Gaga fan site, Gaga Daily, an act which drew a sympathetic message from the pop star on Twitter.
"My little angels! help is on the way," the message read, adding that "haus of gaga techies will be on it ... to the rescue. calling them now."
The site was soon restored.
Anonymous and its offshoot Lulz Security have been linked to a number of high-profile computer attacks and crimes, including many that were meant to embarrass governments, federal agencies and corporate giants.
Spokesmen for NBC Universal declined to comment. An FBI spokesman didn't immediately return a telephone message seeking comment.