Quanto à bomba em si - vejo duas possibilidades:
* Thomas Valente was very high-ranking in the Department of Homeland Security - and one of the men responsible for the attack. It doesn't seem like too much of a stretch for him to use his contacts to bury it before its origin could be confirmed. (Here's his page, with a picture, on the Jericho wikia, in case you've forgotten his face)
* One of the characters was incorrect. According to Wikipedia, the Chinese broadcast in episode 3 mentioned that New York (纽约) was attacked, but in episode 7, Gray Anderson returns and mentions that New York survived. Episode 18 also confirms over radio that New York was spared. This ambiguity could have been 1) a mistake, 2) intentional to keep viewer off-guard, or 3) a clue to a plot element that had to be cut due to the series being canceled (twice).