Federal prosecutors have said they possess a trove of evidence in their terrorism case against Najibullah Zazi, damning accusations set in a powerful narrative. It begins with explosives training in Pakistan, followed by purchases of bomb-making materials in Colorado, experiments in a hotel room and a cross-country trip to New York, which the authorities feared might have been the target of an attack. But interviews with people briefed on the case -- and an examination of court papers filed by prosecutors -- show that much of the evidence presented against 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 FBI agents, 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, indicates that many important facts asserted by prosecutors were discovered after Zazi was told by a Queens imam on Sept. 10 that investigators were looking for him. Moreover, several crucial discoveries were made after 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 FBI agents did not ask about 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 fast-moving nature of the investigation does not necessarily weaken the evidence against Zazi, but it opens a window on what has become clearer since he was indicted last week on a bombing-conspiracy count: that the authorities were forced to react hastily after they began focusing on his activities, and that they were operating in some measure in the dark over the question of how advanced his alleged efforts at bomb-making had become. On Wednesday, a government official briefed on the case said the initial information about a link between Zazi and terrorism was received by counterterrorism analysts late in the summer. The official indicated that analysts had been concerned about Zazi, but the early information did not make clear the nature of the threat he might represent. As the days passed, investigators became more concerned, but huge gaps remained in their understanding of Zazi's activities and any plot. As a result, making decisions about potentially critical investigative steps was difficult, senior law enforcement officials said. 