US Attorney General Eric Holder: "There is a constant threat"
A Pakistan-born US citizen has been charged with terrorism and trying to use a weapon of mass destruction over the failed car-bomb attack in New York.
US authorities said Faisal Shahzad, 30, had also admitted to being trained in bomb-making in Pakistan.
Mr Shahzad was arrested on a Dubai-bound plane at JFK airport on Monday, two days after the car bomb was found.
Earlier, President Barack Obama vowed Americans would "not cower in fear" because of the Times Square incident.
Maybe the US is learning that with all the vigilance, the best intelligence, the tightest security, you can never eliminate all risk, and that the unwitting allies of terrorists are those who want America to be terrified
He said it was a "sobering reminder of the times in which we live" and vowed that justice would be done.
Mr Shahzad is believed to have bought an SUV that was found loaded with an improvised explosive device in Times Square.
Two days after the bomb was discovered, he nearly managed to escape the US.
Despite his name being added to the government's no-fly list earlier on Monday, Mr Shahzad managed to buy a ticket on an Emirates flight to Dubai and made it through JFK's security checks late that evening.
Customs agents checking the passenger list realised Mr Shahzad's name was on it and stopped the flight with just minutes to spare as it taxied to the runway.
Investigators said the Connecticut resident implicated himself and told them he was acting alone.
But court documents stated that he admitted having attended a militant training camp in the lawless Pakistani tribal region of Waziristan.
He apparently told investigators the plot had begun in December last year.
Naturalised US citizen born in Pakistan
Resident of Bridgeport, Connecticut
Reports say he recently returned from five-month visit to Pakistan
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