Who was the first American casualty of World War 1?

Private John Parr (1898 in Finchley - August 21, 1914) was the first British soldier and the first soldier of the Commonwealth killed in World War I.

PRIVATE DUNCAN MCDONALD Camerons listed as missing[on medal index card] 13 August 1914 and is documented as killed in action 13 August 1914[CWGC and Soldiers died in the Great War] possibly died on boat to France,or actually went missing deliberately[he was 16 years old], killed by accident or the date is a mistake? The date is common throughout the reference data showing him killed in action 13 august 1914.

He is on the Memorial to the missing at LA FERTE-SOUS-JOUARRE Seine-et-Marne, France also showing him to have been killed in action 13 August 1914 aged 16

The 1901 census confirms he was indeed just 16.

Also I believe there was a Royal Flying Corps casualty before 21 August 1914