"He was told to be very careful about not letting anything track back to him. No receipts, and no paper. No nothing," an official source close to the investigation was quoted as saying by the Los Angeles Times.
The Pakistani Taliban had instructed Shahzad to always pay cash and never ask for or receive receipts for his transactions. "He was told to leave no paper trail at all," it said.
The paper said Shahzad had paid in cash for his gun, as also for the van he bought from a Connecticut-based teenager.
The investigators probing the plot are trying to determine how much Taliban money was provided to front the operation, who put up the funds and how it was paid out to Shahzad, the report said.
Another report in the same paper has said that Shahzad, a naturalised citizen of the United States, might have been disturbed by increasing US drone attacks in Pakistan's tribal belt.
Agents interviewing the 30-year-old have learned that he was upset over repeated CIA drone attacks on militants in his the country of his birth.
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