"I actually lost my brother yesterday to a brain tumor and I couldn't imagine being anywhere else right now," Cook, 26, told the crowd at the benefit for brain cancer research (see video here). Cook was both serving as grand marshal and running with a team that raised $98,000.
Cook's brother, Adam, 37, had battled the disease throughout Cook's American Idol competition, and had the chance to see his younger sibling perform in April 2008 on the famous stage. At the time, Adam, a married father and lawyer in Terre Haute, Ind., was undergoing chemotherapy treatment.
"I'm not going to lie to you. He still has a tough road ahead of him," Adam's longtime friend Darrick Scott had told PEOPLE. "But this trip to Los Angeles has put a whole new pep in his step."
The Idol judges had even noted that having Adam in the front row may have given David the inspiration needed to win.
Simon Cowell on Sunday offered his condolences. “I was incredibly saddened to hear the news," he said in a statement to PEOPLE. "I met Adam Cook a few times at American Idol. He was an incredible guy, and right now my thoughts are with David and his family.”
In March this year, the Idol champ Cook canceled two shows on his current tour "to deal with personal family matters." His rep had told PEOPLE that he had recently been spending time with his brother.
For Sunday's race, the singer flew into the capitol after a performance Saturday night at Sun Fest in Palm Beach, Fla., where he had spent most of the day hanging out. His plan was to take a 3 a.m. flight.
"I don't run normally, but it seems a small sacrifice compared to what those who have fought cancer have endured," Cook earlier had told PEOPLE. "And the fact that my fans have rallied to raise so much in donations is the biggest compliment."
At the early-morning benefit, he said, "I lost one today," addressing all the runners, supporters and organizers, "but I gained 9,000 [friends] and I will be here every year that they will have me."