Federal prosecutors have said they possess a trove of evidence in their terrorism case against Najibullah Zazi, a set of damning accusations laid out in a powerful narrative. But interviews with people briefed on the case - and an examination of court papers filed by prosecutors - show that a great deal of the evidence presented against Mr. Zazi was not the result of a lengthy investigation. Instead, much of it was collected on the fly in the last two weeks, with hundreds of F.B.I. agents, federal prosecutors and detectives rushing to fashion a mosaic of details into a case that could be brought to court. The review of the government's presentation, which is largely contained in a preliminary court document filed last week, suggests that many important facts asserted by prosecutors were discovered after Mr. Zazi was told by a Queens imam on Sept. 10 that investigators were looking for him. Moreover, several crucial discoveries were made after Mr. Zazi, a 24-year-old airport shuttle bus driver, had returned on Sept. 12 to Colorado, with his mission, if he had one, aborted. Some store and hotel employees in the Denver area said F.B.I. agents did not ask about Mr. Zazi's purchases of beauty salon products that contained the raw materials to make explosives or his stay in a hotel suite to mix them until Sept. 17, five days after his return to Colorado. The lawyer who appeared beside him in Brooklyn, J. Michael Dowling, said after the hearing that prosecutors could not secure a conviction of his client on the conspiracy charge based solely on the evidence presented in the detention memorandum, because it contained no proof that he conspired with anyone to commit a crime.