Had the bomb planted in Times Square by Faisal Shahzad exploded it "would have killed thousands of people", a secret FBI test of a correctly made version of explosive has revealed.

A replica of the bomb was exploded outside Harrisburg, Pennsylvania to test its destructive force, sources said.

The New York Post quoted law enforcement sources as saying that had Shahzad built the device the way he had originally intended to, it would have turned his SUV and nearby vehicles into a fatal spray of razor-sharp fragments and transformed building windows into glass guillotines hurtling to the streets, cutting down hundreds of people walking by.

The tests show the bomb would have dwarfed the Oklahoma City blast.

"It would have been the biggest thing ever to happen in this country since Sept. 11," another source said.

"It definitely would have been bigger than [the 1995] Oklahoma City" bombing of the federal building that killed 168 people, the source said.

He added: "There would have been a lot of casualties. People would also have been stomped to death from running away. It would have been panic. The buildings would have been severely damaged."

Thankfully, street vendors noticed the smoke and alerted police.

The cops quickly evacuated the tourist-packed area as they dismantled the device.

Shahzad, 30, was nabbed several days later trying to flee the country. Last month, he pleaded guilty to federal terrorism charges, saying he wanted to attack America in retaliation for US military killings of Muslims abroad. (ANI)