regex alternation problem

[Jesse Aldridge]

import re

s1 = "I am an american"

s2 = "I am american an "

for s in [s1, s2]:
    print re.findall(" (am|an) ", s)

# Results:
# ['am']
# ['am', 'an']

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I want the results to be the same for each string.  What am I doing
wrong?