"He tried to lessen the explosive nature of the fertiliser that was used because he thought he would get a higher profile as he went to buy it," New York's police commissioner Ray Kelly was cited as saying by the Wall Street Journal.
A secret FBI test of a correctly made version of the Times Square bomb revealed that it "would have killed thousands of people" if it had been made to explode as terrorists had intended, the New York Post said citing law-enforcement sources.
Had he built the Times Square device the way he had originally intended to, Shahzad 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 results were discovered after the FBI built its replica of the bomb Shahzad set out to make-with the exact components he had initially intended to use - and exploded it last month outside Harrisburg, Pennsylvania, to test its destructive force, the Post said.
The results of the explosive test were sobering - showing that Shahzad was on track to becoming the biggest individual mass murderer in US history, several sources said.
Meanwhile, Kelly during a discussion Tuesday at the Centre for National Policy, a Washington think tank, said a key question in the early stages of the investigation had been how a trained terrorist could craft such a poorly made bomb, consisting of weak fireworks, propane tanks and non-explosive fertiliser.
Shahzad also used M-88 fireworks that were much weaker than other alternatives, the Journal cited Kelly as saying.
Law enforcement agencies, particularly the Federal Bureau of Investigation, maintain a number of "tripwires" designed to encourage people who sell everyday products that could be used to make explosives to notify agents of any suspicious behaviour or purchases.
Shahzad was apparently so worried about the tripwires that he deliberately built a weaker, less effective bomb, the Journal said.