They were probably wearing military outfits - but the worst thing is that perhaps they were Americans. Or at any rate, people who looked American enough so that nobody would have had alarms go off when they looked at them.
Odd that there are no further descriptions.
Look American enough? Anyone can look American enough.
In my boot camp there were some of every flavor, some with
such contorted names they were called ‘alphabet’.
We even had a Russian. And this was in 1969.