There are places, like the medical examiners office in Tucson, Arizona, where the human cost of migration to the United States is counted.

Dr Bruce Parks, the chief medical examiner, is a tall, thin, quiet man. He put on rubber gloves and led me over to a zippered plastic bag sitting on a steel gurney.

"These people, we don't know who they are, and we may never know," he says, opening the container. 

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