In a previous life I worked for the US Dept. of the Interior. (Don't ask - you'd only laugh!).
The requirements then were for 'Federal' employees to be US Citizens. (I worked on an exchange, so I didn't count!). To become such you had firstly to become a 'Registered Alien'.
Following a period of 5 years being a model 'wannabe' citizen, (No speeding tickets, etc.), you could then APPLY for US Citizenship: " I pledge allegiance to the flag ......", "Oh say can you see, by the dawn's ...." etc. etc. etc.
The US Immigration & Naturalization Service web site would no doubt give you all the info.
Q 4 Scott V: Is the above still the case?
Rgds
T3