Country House, a 65-1 shot, finished second in the slop before an objection was raised, causing a lengthy delay while stewards repeatedly reviewed several angles of video footage, before he was elevated into the winner's circle.
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The stunning outcome gave Hall of Fame trainer Bill Mott his first Derby victory at age 65. Jockey Flavien Prat, who originated the claim of foul, also won his first Derby.
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Country House paid $132.40 to win - the second-highest payout in Derby history.
It was a crushing turn of events for trainer Jason Servis and jockey Luis Saez, who already had begun celebrating what they thought were their first Derby victories.
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Instead, Maximum Security was dropped to 17th of 19 horses. The colt was the 9-2 second choice in the wagering.
Prat claimed that Maximum Security ducked out in the final turn and forced several horses to steady.
War of Will came perilously close to clipping heels with Maximum Security, which could have caused a chain-reaction accident.
VIDEO: Maximum Security works out days before Derby
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Maximum Security works out ahead of Kentucky Derby
The stewards reviewed race footage for nearly 20 minutes while keeping the crowd of 150,729 in suspense, clutching their betting tickets. Trainers and jockeys involved stared at the closest video screen waiting for a result.
Code of Honor was moved up to second and Tacitus was third.
Improbable was fourth and Game Winner fifth, two of trainer Bob Baffert's trio of entries.