One of the alarm clocks found in a bomb in a Nissan Pathfinder sports utility vehicle in Times Square on Saturday evening is shown in this New York Police Department photograph released on May 2, 2010. [Xinhua/Reuters]

The suspect bought the vehicle with cash, and the sale was handled without any formal paperwork. Officials said that the sloppy transactions practiced by the plotters suggested that the terrorist attempt was "hastily conceived."

Investigators are trying to find motives behind the plot as they started to explore whether Shahzad or others involved in the plot had been in contact with people or groups overseas.

Taliban operatives in Pakistan appeared to have mentioned the bomb plot and have claimed responsibilities in three videos released on the Internet after the incident. But there has been evident supporting these claims. Investigators didn't know whether Shahzad is linked to Taliban.

White House press secretary Robert Gibbs has already denounced the plot as an act of terrorism.

On Monday, the international terrorism branch of the Joint Terrorism Task Force, a multi-agency group led by the Justice Department, has taken over the investigations from New York police.