(CNN) -- Faisal Shahzad, the 30-year-old Pakistani-American who has pleaded guilty to the botched Times Square terror attack, altered the composition of the bomb he intended to use to avoid being detected by authorities, according to the head of the New York City Police Department.
Shahzad "tried to lessen the explosive nature of the fertilizer that he used because he thought that he would get a higher profile if he went to buy it," Police Commissioner Ray Kelly told CNN on Tuesday. "So he sort of dumbed that down."
At the same time, Shahzad "substituted explosives by using [more readily available] firecrackers. ... He was concerned about surfacing in that regard" as well, Kelly said.
"He went to Pennsylvania to buy them. So those two things -- the explosive and the fertilizer -- did not follow the initial plan, so to speak."
Fortunately, changing the "ingredients ... made it very difficult for [the bomb] to go off," Kelly said.
In June, Shahzad entered guilty pleas in federal court to all 10 counts he was facing in connection with the attack. He was accused of attempting to set off the vehicle bomb in Times Square on May 1.
He was arrested two days later while trying to leave the country on a flight from New York's John F. Kennedy International Airport.
Shahzad has said he traveled to Pakistan in December to join the Taliban. While there, he said, the Taliban gave him $4,000, which he combined with $4,900 of his own money to pay for the materials used in the attempted attack.
The bomb, had it detonated, would have been "extremely deadly," Kelly said.
Shahzad's plan, Kelly noted, was to have the bomb go off "right across from a major hotel."
"If you go to New York City right now -- certainly on a Saturday night -- you just see how densely populated it is," Kelly said. "I can tell you there's more people [in Times Square] now than there's ever been. ... Tourism is up. It's come back in New York. And [Shahzad] said that he went there on Saturday night to kill the maximum amount of people."
Shahzad, who has said he considers himself "a mujahedeen and a Muslim soldier," is facing a penalty of life in prison. He is scheduled to be sentenced on October 5.
CNN's Carol Cratty contributed to this report.