Stalker,
Undefined,
17 years ago
That really depends on how much information Lex gave out about the creatures after the events of the first movie, as she was the only survivor.
Either way, I doubt that anybody would really know what they were up against, as in the Alien films, which occured many years in the future, the creatures had, according to reports, never been discovered before, so even if the Government had information about the Xenomorphs, they would have kept it tightly under wraps.
They clearly had some basic knowledge of the predators though, as they sent Peter Keyes' team to capture the lone hunter in L.A. during the events of Predator 2, 14 years before the incident in Antarctica.