The official, who spoke on condition of anonymity because the investigation continues, confirmed that investigators know the identities of at least three people believed to be in on a bombing plot they say might have targeted mass transit in the New York area.
Authorities released a flurry of terrorism warnings for sports complexes, hotels and transit systems even while saying the plot was disrupted before it become an immediate threat. But many questions remain unanswered, including the whereabouts of co-conspirators and whether any may be cooperating with the probe.
There also have been no reports that any of the bomb-making materials have been recovered.
The accomplices are suspected of traveling from New York City to suburban Denver this summer and using stolen credit cards to help Najibullah Zazi stockpile beauty products containing hydrogen peroxide and acetone, which can be key ingredients for homemade bombs, authorities have said.
Before the raids, police detectives showed a source - a Queens imam at a mosque where Zazi had once worshipped - photographs of him and three people considered possible suspects, court papers say. It was unclear whether those three were the same ones suspected of traveling to Denver.
The official who spoke to The Associated Press declined to comment further Monday. Spokesmen James Margolin for the FBI, Edward Mullen for the New York Police Department and Robert Nardoza for the U.S. attorney's office in Brooklyn declined to discuss the case.
After initially being charged along with his father and the imam with lying to investigators, Zazi was due in federal court in Brooklyn on Tuesday for an arraignment on charges he conspired to use weapons of mass destruction. The 24-year-old airport van driver has denied any wrongdoing.
A letter filed by Brooklyn prosecutors last week argued that that Zazi should be jailed indefinitely because, as an Afghan immigrant with ties to Pakistan, he could flee, and because he "poses a significant danger" to the community.
Evidence gathered so far - including bomb-making instructions found on his laptop computer - shows "that Zazi remained committed to detonating an explosive device" until he was arrested, the letter said.
Prosecutors allege that Zazi has admitted that while living in Queens, he traveled last year to Pakistan and received explosives training from al-Qaida. Security videos and store receipts show that when he returned and moved to Aurora, Colo., he and three others bought several bottles of beauty products over the course of several weeks, court papers said.
On Sept. 6, Zazi took some of his products into a Colorado hotel room outfitted with a stove on which he later left acetone residue, authorities said. He repeatedly sought another person's help cooking up the bomb, "each communication more urgent in tone than the last," the papers said.
The FBI was listening to Zazi and becoming increasingly concerned as the anniversary of the Sept. 11, 2001, attacks and a New York visit by President Barack Obama approached, officials said. They decided to track him on Sept. 9 when he rented a car and drove to New York.
On Sept. 10, Zazi told the Queens imam in an intercepted phone call that he feared he was being watched, court papers said. The imam later tipped Zazi off, saying police had come around and asked questions, the papers said.
Zazi cut a five-day trip short and flew back to Denver on Sept. 12. He was arrested a week later.