Unnamed Obama administration officials were quoted by the Washington Post as saying that it increasingly appeared that several people were involved in the plot, possibly with international links.
And the White House called the incident an attempted act of terrorism for the first time on Monday, with spokesman Robert Gibbs saying: "I would say that was intended to terrorise, and I would say that whomever did that would be categorised as a terrorist."
But officials have cautioned that foreign links do not necessarily mean al-Qaeda was involved.
On Sunday the Pakistani Taliban said it planted the bomb to avenge the killing in April of al-Qaeda's two top leaders in Iraq.
But Ray Kelly, the New York police commissioner, and Michael Bloomberg, the city's mayor, said there was "no evidence" to support that claim.
Police and the FBI continue to pore over "hundreds of hours" of security videotape from around Times Square, looking for a white man in his 40s spotted on a surveillance video near the vehicle containing propane gas cylinders, petrol cans, fertiliser, fireworks and timing devices.
One source told the Reuters news agency the fact that the man on the video did not match the description of the car buyer gave "some credibility to the statement that [investigators] believe there is more than one individual involved and that it is international in scope".
|The parked vehicle was rigged with a crude homemade bomb [Reuters]
Eric Holder, the US attorney-general, sought to reassure Americans on everything possible was being done to find the culprits.
"I think we have made substantial progress," he said in Washington DC on Monday. "We have some good leads.
"I can assure the American people this investigation is solid."
Police shut down several streets, deployed bomb-squad personnel and evacuated thousands of people from the heart of Manhattan after a street vendor alerted police that he had seen smoke emerging from the parked vehicle late on Saturday.
A homeland security official said security had been tightened at airports on the US east coast following the Times Square incident.