Even though federal investigators had been keeping their eye on former Queens street vendor Najibullah Zazi for months, it was only the last-minute scramble for damning clues after his September visit to New York that provided them with enough evidence to charge him with a terror conspiracy. While his lawyer argues that there's no conspiracy (no one else has been charged with conspiring alongside Zazi), in the two weeks before his arrest the FBI made a lot of progress wrapping up their case. It was only after Zazi was alerted by a double-crossing double agent on September 10 that the investigation kicked into full gear — which meant that there was not enough evidence yet compiled to stop him from returning to Denver from Queens. After that point, investigators quickly found bomb-making guides on his laptop, surveillance video and receipts indicating he'd purchased a dozen bottles of beauty products containing hydrogen peroxide (a key element to the bombs), and evidence that he'd been cooking up explosive materials in a hotel room.

Rush for Clues Before Charges in Terror Case [NYT]