All of those former Iraqi Republican Guardsmen were people who were given asylum after Desert Storm so it is not hard to believe that they were closely watched by the Feds. They didn't sneak into the U.S. they were here legally.
How did they manage to work with McVeigh and Nichols without the knowledge of the gov?
No Iraqi Republican Guardsmen were given asylum. What they did was take off their uniforms and blend into the general population.
The ones we brought back were supposedly the ones that had opposed the government and were therefore on our side and likely to be harmed if left behind.
The problem was (as usual) we did a crappy job of vetting. Lot’s of bad guys were brought over here thinking they were good guys, while lots of actual good guys were left over there.
And NO government agency did any kind of watching. There is some number (I believe it is about two and a half thousand) that remain unaccounted for today.