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The DMAA-containing performance-enhancing supplement Jack3d has come under the microscope after it was revealed Wednesday that marathoner Claire Squires died last year after using it. Squires' death at age 30 shocked the global running community last April when she collapsed a mile from the finish line of the 2012 London Marathon. Her death remained unexplained until Wednesday, when coroner Philip Barlow told a court that she had ingested Jack3d before competing in the race, and that the supplement was the most likely cause of her death, according to the Guardian newspaper.