(CNN) -- It's hard to find a bigger Justin Bieber fan than Hailey Roser.
The 4-year-old Wisconsin girl has two life-size cardboard cutouts of the Biebs in her hospital room, where she fights a daily battle against an aggressive form of brain cancer.
She has endured two brain surgeries -- but doctors can't remove all of her brain tumor. Facing months of radiation and chemotherapy, Hailey wanted nothing more than a visit from the pop superstar.
After a social media blitz, including the viral Twitter hashtag #justinvisithailey, a radio station helped score meet-and-greet tickets to Bieber's concert Sunday in Milwaukee.
But then a mass shooting at a suburban salon Sunday forced the hospital to go on lockdown -- meaning no one could enter or leave.
This is the story of a long cherished visit that almost didn't happen.