The FBI has detonated a working replica of the car bomb used in the failed Times Square terror attack, creating a large explosion that destroyed other vehicles and scattered flaming debris.
The test in central Pennsylvania showed that the home-made bomb, had it been built and detonated properly, would have killed or wounded an untold number of pedestrians and damaged buildings along the street where the car was abandoned by Faisal Shahzad on May 1, officials said.
"It would have been extremely deadly," New York police commissioner Raymond Kelly said when asked about the test after an event at a Washington think-tank.
Mr Kelly did not go into specifics about the FBI test. But two other officials said it was conducted late last month in a remote area 30 miles outside State College, Pennsylvania, and that a video of it was played for a gathering of authorities earlier this week.
The test was first reported in the New York Post, which quoted an unnamed source saying that the results suggested the bomb could have been more deadly than the 1995 Oklahoma City car bombing that killed 168 people.
But one of the officials said the Oklahoma City bomb - also made of fertiliser - was about 10 times larger than the one left in Times Square.
Calling himself a "Muslim soldier", Shahzad, 30, pleaded guilty on June 21. During his plea hearing, he traced his plot to a 2009 trip to Pakistan, where he said he received explosives training and funding from the Pakistani Taliban for his one-man scheme.
He returned to the US and loaded a 1993 Nissan Pathfinder with a fertiliser-fuelled bomb packed in a gun cabinet, a set of propane tanks and gas canisters rigged with fireworks that he hoped would cause a chain-reaction explosion.
Instead, authorities say the bomb malfunctioned, emitting smoke that attracted the attention of a street trader, who notified police.