Chris Nemeth, a clerk at Phantom Fireworks' Matamoras, Pa. store said he sold Shahzad eight boxes - $95 worth - of fireworks on March 8.
He (Shahzad) acted like a "calm, unremarkable customer," Nemeth told CBS' "The Early Show" Friday. Nemeth said Shahzad produced a Connecticut driver's license with his name on it and paid with a $100 bill.
"The thought never crossed my mind" that someone would try to use these fireworks as part of a terror attack, Nemeth said.
"The perpetrator purchased Phantom's Silver Salute M-88 brand firecrackers. The perpetrator apparently hoped for a chain ignition and mass detonation of the firecrackers. Consumer firecrackers do not have enough power to produce such an effect, and as a result they do not mass detonate," the fireworks company said in a press release Thursday.
"We are very disappointed this coward would use a Phantom Fireworks product intending to cause harm to Americans," the company added. "We are thankful there was no injury or damage in this event, and we are gratified that we were able to offer something of value in this investigation."
Company President Bruce Zoldan had said Wednesday that "the M-88 he used wouldn't damage a watermelon." Nemeth agreed, saying the Silver Salute fireworks Shahzad bought would do nothing but produce a "gentle pop."
Watch surveillance tape of Shahzad browsing for fireworks:
More coverage of the Times Square Bomb Plot:
Shahzad Practiced Bomb Plot, Official Says
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Shahzad: Number Cruncher to Terror Suspect
Bomb Suspect Shahzad Seen Walking Away in Video
How You End Up on the U.S. No-Fly List
Shahzad Bought Weak Fireworks, Store Owner Says