Current and former U.S. officials say the allegations in the case embody their worst fears — that a legal U.S. resident could quietly leave the country, receive explosives training from al Qaeda in a lawless region of Pakistan, then return to U.S. soil…

U.S. officials caution that there is much they don’t know about Mr. Zazi and the other suspects. They also say the current case contains allegations that are highly unusual since 9/11. It appears to be only the second time a U.S. citizen or permanent resident is alleged to have traveled overseas after 9/11 to train with al Qaeda, then come back to live in the U.S. Also, many previous cases were built around paid government informants, which wasn’t the case here, officials said…

Two U.S. officials said key allegations in the case are more reminiscent of the London and other European plots than any other case seen to date in the U.S. “This appears to be different than any of the previous cases because it looks very similar to what we’ve seen recently in Europe,” one of the officials said.