The use of "not" in idiomatic English
If I understand the Pentagon's attitude correctly, the sentence should say "Pentagon experts on Friday said it was impossible to imagine that the missile could have been fired without Russian help," without the not. This is an example of misnegation (or overnegation), which Mark Liberman of Language Log describes as "the confusions arising from our poor monkey brain's inability to deal with combinations of negations, modals, and scalar expressions." See that link for examples and further explanations.