Zazi intensified his bomb-making experiments this month, cooking up substances in a Colorado hotel suite he rented on Sept. 6-7 before driving 1,600 miles to New York over the course of about two days. He became aware that law enforcement was onto him when he was stopped entering the city on Sept. 10, causing the plot to unravel.
"He was asking for information on flour and how to get the contents right," Neff said in court.
The Metropolitan Transportation Authority -- which runs New York City's subway system, buses and commuter rails -- declined to comment on the revelation of a Sept. 11-timed plot. It reissued a statement from earlier in the week that it has boosted its police presence at "key commuter rail locations" since the terror threat became public.
Federal agents and police officers in New York visited up to 200 locations a day in the area during the probe, including beauty-supply stores, extended-stay hotels that have rooms with kitchens, hardware stores, truck rental agencies and storage facilities.
Zazi was scheduled to appear in federal court Tuesday in Brooklyn.
A government request to deny bail laid out a chronology of the alleged scheme, which prosecutors said had been in the works for more than a year.
On Sept. 6 and 7, Zazi checked into a suite at a Colorado hotel with a kitchen and a stove, government papers say, and tried to contact an unidentified associate "seeking to correct mixtures of ingredients to make explosives."
"Zazi repeatedly emphasized in the communications that he needed the answers right away," the papers said, adding that each communication was "more urgent than the last."
Beauty supply store employees in New York and the Denver suburbs said authorities had been asking whether anyone had come in buying a lot of hydrogen peroxide or acetone.
At Beauty Supply Warehouse in suburban Denver, Paul Phillips said a co-worker told investigators he had sold chemicals to Zazi. Company President Karan Hoss said the firm turned over security video of a man matching Zazi's description to the FBI. A check of sales found that someone bought a dozen 32-ounce bottles of a hydrogen peroxide product in July. More was purchased in late August, Hoss said.