Guatemala is still plagued by urban crime, but it is peaceful now compared to the decades of bloody civil war that convulsed the small Central American country. As he arrives in the capital, Guatemala City, FRONTLINE/World reporter Clark Boyd recalls, “When the fighting ended in the 1990s, many here wanted to move on, burying the secrets of the war along with hundreds of thousands of the dead and disappeared. But then, in July 2005, the past thundered back.”

In late 2006, a statistical study of deaths that occurred after the invasion of Iraq ignited a storm of controversy. This Lancet study estimated that more than 650,000 additional Iraqis died during the invasion than would have at pre-invasion death rates, a vastly higher estimate than any previous. But in January, a World Health Organization study placed the number at about 150,000.

International human rights work attracts activists and lawyers, diplomats and retired politicians. One of the most admired figures in the field, however, is a ponytailed statistics guru from Silicon Valley named Patrick Ball, who has spent nearly two decades fashioning a career for himself at the intersection of mathematics and murder. You could call him a forensic humanitarian.

The tension started in the witness room. “You could feel the stress rolling off the walls in there,” Patrick Ball remembers. “I can remember realizing that this is why lawyers wear sport coats – you can’t see all the sweat on their arms and back.” He was, you could say, a little nervous to be cross-examined by Slobodan Milosevic.