Justin Guarini, season 1 runner-up, confirmed the news on his Instagram:

@realnikkimckibbin was a fiery, funny lady who could sing the Hell out of a rock song with the same kind of ease and command she lovingly used to cut you with her twangy Southern wit. 

Even in our 20's when we were on American Idol together I could tell that she'd had a challenging life, and that not that many people had been kind to her along the way…

…but I'll never forget the day that her idol, Stevie Nicks, sent her flowers with a card that said "You are the Gypsy that I was…". Nikki was on cloud nine and the joy and excitement that radiated from her was infectious. After the kind of emotional beating she took at the hands of the masses, she deserved to feel good about herself, even for a little while. 

Rest well, "Gypsy"…and thank you for the laughter, merciless teasing, strength, vulnerability, love and friendship you showed me during our time together in the spotlight. #rip #americanidol

Nikki's first foray into reality TV won on the WB competition, Popstars in 2001. But it was American Idol that made her a celebrity. Her  cover of Alannah Myles "Black Velvet" for the Top 3 round, was her best remembered performance on Idol. But she also covered Stevie Nicks, Alanis Morissette, Melissa Etheridge, Janis Joplin, Bonnie Tyler and more on the show.