Who was the first American diplomat to die in the line of duty?

The answer is:

William Palfrey of Massachusetts was the first U.S. consul, but was lost at sea on his way to his post. His name is the first on the memorial plaque in the lobby of the Department of State that honors U.S. diplomats who lost their lives under heroic or tragic circumstances.

Yesterday's question:

What was the longest term of any Secretary of State?

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