A metal rifle cabinet placed in the SUV's cargo area was packed with fertilizer, but NYPD bomb experts believe it was not a type volatile enough to explode like the ammonium nitrate grade fertilizer used in previous terrorist bombings.
Police said the SUV bomb could have produced "a significant fireball" and sprayed shrapnel with enough force to kill pedestrians and knock out windows.
A vendor alerted a police officer to the parked SUV, which was smoking. Times Square, clogged with tourists on a warm evening, was shut down for 10 hours. A bomb squad dismantled the explosive device, and no one was hurt.
But Holder said Americans should remain vigilant.
"It's clear," he said, "that the intent behind this terrorist act was to kill Americans."
Authorities did not address Shahzad's plans in Dubai. The airport there is the Middle East's busiest and is a major transit point for passengers traveling between the West and much of Asia, particularly India and Pakistan.
Dubai-based Emirates airline said three passengers were pulled from Flight EK202, which was delayed for about seven hours. The airline did not identify Shahzad by name or identify the other two passengers.
The aircraft and passengers were then re-screened before taking off Tuesday morning, and the airline is "cooperating with the local authorities," Emirates said in a statement e-mailed to the AP.
In Pakistan, Interior Minister Rehman Malik told the AP that authorities had not been formally asked for help in the probe but would cooperate if asked.
More than a dozen people with American citizenship or residency, like Shahzad, have been accused in the past two years of supporting or carrying out terrorism attempts on U.S. soil, cases that illustrate the threat of violent extremism from within the U.S.
Among them are Army Maj. Nidal Hasan, a U.S.-born Army psychiatrist of Palestinian descent, charged with fatally shooting 13 people last year at Fort Hood, Texas; Najibullah Zazi, a Denver-area airport shuttle driver who pleaded guilty in February in a plot to bomb New York subways; and a Pennsylvania woman who authorities say became radicalized online as "Jihad Jane" and plotted to kill a Swedish artist whose work offended Muslims.
Contributing to this report were Associated Press writer Pete Yost in Washington, AP Video journalist Ted Shaffrey in Bridgeport, Conn., AP photojournalist Doug Healey in Shelton, Conn., and AP writers Chris Brummitt in Islamabad, Adam Schreck in Dubai and John Christoffersen in Shelton, Conn.