Investigators told CBS News that agents are aggressively following a steady stream of leads in the case and have essentially constructed a family tree with 24-year-old Najibullah Zazi at the center, at least 12 of his known "disciples" in a ring around him and other associates branching out from there.
Zazi, his father and Muslim religious leader Ahmad Wais Afzalion charges they lied to the FBI but weren't charged with terrorism, and the scale and scope of the plot remains unclear. They have denied the charges against them.
Law enforcement officials said Zazi may have been plotting with others to detonate backpack bombs on New York trains in a scheme similar to the attacks on the London subway and Madrid's rail system in the last few years. Backpacks and cell phones were seized in raids on apartments Zazi visited in New York.
America's top law enforcement officer believes that threat, reported CBS News chief investigative correspondent Armen Keteyian.
CBS News learned Tuesday that a hunt was on to see if suspects had stored a cache of explosives somewhere in New York City. In an exclusive interview with "60 Minutes," Attorney General Eric Holder told correspondent Steve Kroft just how urgent the threat was.
"I think we've disrupted that which they've planned and it's not totally clear to us at this point what it is they had in mind, though I think it is clear that something very serious and something very organized was under way," Holder said.
Zazi had been under FBI surveillance, but the criminal complaint against him suggests NYPD officers acting without the bureau's knowledge may have blown their cover by questioning Afzali, who is an imam in the New York City borough of Queens.
"They came to ask me about your characters," Afzali told Zazi in a secretly recorded Sept. 11 telephone conversation. "They asked me about you guys."
At least one of those New York Police Department detectives, referred to in the recently unsealed criminal complaint, works for a division that operates independently from an FBI-run terrorism task force.
The complaint also suggests investigators may have tipped off Zazi, a 24-year-old Denver airport shuttle driver, by towing and searching a rental car he was using on a New York City trip that heightened fears of an attack.
The maneuver, authorities say, produced evidence of bomb-making instructions retrieved from a hard drive on Zazi's laptop.
But it also apparently didn't get by the suspect: In the phone conversation with Afzali, Zazi said the car's disappearance convinced him he was being watched.
NYPD and FBI officials have denied that the potential missteps forced their hand in a series of high-profile raids last week, prompted Zazi to abort his New York visit and caused friction between the two agencies, which work together through the Joint Terrorism Task Force.
NYPD Commissioner Raymond Kelly, asked Tuesday if he had any concerns about the handling of Afzali, declined comment on the investigation beyond what was in court papers, saying the probe was classified.
NYPD spokesman Paul Browne insisted the NYPD and the FBI "worked closely and successfully in this case and in scores of others." He declined further comment.
There were no new arrests Tuesday in the terror probe, but sources tell CBS News that up to a dozen people continue to be closely monitored by the FBI.
In a statement, the FBI and the Department of Homeland Security said that while the agencies "have no information regarding the timing, location or target of any planned attack, we believe it is prudent to raise the security awareness of our local law enforcement partners regarding the targets and tactics of previous terrorist activity."
Afzali's attorney, Ron Kuby, has said his client had a history of giving police information as a community liaison and religious leader in his Queens neighborhood. Kuby claimed Afzali was doing their bidding by talking to Zazi and finding out what he was up to.
"My client is being blamed for an investigation botched by the authorities," Kuby said Tuesday. "It's much easier to blame some obscure Afghan imam."
The complaint, filed in federal court in Brooklyn, says NYPD detectives first visited Afzali at his home on Sept. 10.
Around that time, the public was unaware that federal authorities were tracking a suburban Denver man with possible links to al Qaeda who had driven to New York City - Zazi. The complaint says that unnamed detectives showed Afzali photos of Zazi and that Afzali admitted he recognized him.
Kuby said one of the detectives was his client's usual police contact, an investigator assigned to the police department's Intelligence Division, not the terrorism task force.
The day after police spoke to Afzali, the FBI intercepted his phone call with Zazi discussing the NYPD's inquiry. The next day, Afzali's lawyer said, his client had his first-ever contact with the FBI, when he agreed to answer questions at their Manhattan headquarters.
On Sept. 14, Afzali also agreed to a search of his home, then gave DNA samples and a written statement on Sept. 17, the attorney said.
Afzali was arrested on Sunday on charges he lied in the statement by denying that he had tipped off Zazi.