Adam Cook, a lawyer, was 36. He had struggled with the disease for 11 years.
On Sunday morning, David made the announcement of his brother's death at the 12th Annual Race for Hope 5K run/walk in Washington, D.C., which benefited finding a cure for brain cancer. He served as the event's honorary chairman.
"I actually lost my brother yesterday, to a brain tumor," he told a stunned crowd in a video of the event posted on YouTube.
"Idol" fans may remember Adam from his emotional appearance last year on the reality competition, when he watched David perform Mariah Carey's "Always Be My Baby" in the audience. David went on to win the contest during the show's seventh season, beating out competitor David Archuleta.
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