We first need to know what sort of specialty Army Capt. Andrew Clements' was providing to the Army.
Was he active or reserve, in intel or a combat MOS?
Where was he stationed?
If we know these things then we'll be alot further along then we would if we all keep sitting around wondering about spooky stuff.
He was a Field Artillery Officer and was "officially" on his way back east after finishing post-graduate work in Monterey, Ca where he was studying business and public policy.
He was newly assigned to the U.S. Army Testing and Evaluating Command Center.
I'd venture a guess that he was either working on something shady or was transporting information that the Pentagon deemed sensitive or secret.
I'd also venture a guess that this Captain Clark was either an Army Officer....but I doubt that was his real name....or maybe even a spook for the CIA, NSA or DIA.