The Daily Mirror recently used the phrase "Pentagon experts on Friday said it was impossible to imagine that the missile could not have been fired without Russian help". This exact phrase has appeared in other news stories as well, likely spread from the original Pentagon press release via newswire.
Is the "not" a mistake or idiomatic English? Without the "not" the phrase makes sense to me and the meaning is consistent with the context. With the "not" I don't even know what it means.