The attack during the Monday morning commute marked what experts called a "big first" in the fight against terrorism.
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Both the 2009 underwear bomber and 2001 shoe bomber attempted suicide bombings on planes bound for the United States, but this attack marked the first true suicide bombing attempt on U.S. soil, said Terrorism Expert Manuel Gomez, a former NYPD sergeant and FBI special agent.
"That's a game changer," Gomez said. "If somebody is out there, going to the Internet to make a low-sophisticated pipe bomb, and his or her goal is to go out there and kill as many people as possible, that is very difficult to stop in the act of doing it. That needs to be prevented well beforehand at the planning stages, which I believe someone had to know this person was planning to do something like this."
Law enforcement sources described the device as a low-tech explosive made in part using a roughly 12" pipe, black powder, a 9-volt battery and wire from a Christmas light; and filled with nails and other bits to cause additional injuries.
Ullah strapped it to his body, said NYPD Deputy Commissioner for Counterterrorism and Intelligence John Miller, using Velcro and zip ties.
"This device did not fully explode," said former NYPD Commissioner Ray Kelly. "If it had it would have killed him."
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While Ullah has made statements that he intentionally detonated the device, security experts have called the location of the attack peculiar, and questioned whether Ullah truly intended to detonate the device in a subway passageway near Times Square.
"I highly doubt that he wanted to detonate this device in the passageway because he wanted to get the biggest bang for his buck," Gomez said. "So, I think he was either going to go on a bus or go on a subway train that was loaded with people and try to detonate the device there."
"There is a possibility that it might have been accidentally detonated, but he doesn't want to admit that," Kelly said.
Police said this kind of attack underscores the importance of early detection and prevention. Following the incident, the NYPD put out a series of reminders on social media saying, 'If you see something, say something."
Officers will examine Ullah's activities online, on social media and in the community to further determine how this plot came together.