"I believe he went through some training," says Cavanaugh, who has followed the case and looked at pictures of the bomb and diagrams released by the FBI like the one below. "[But] I would venture to say, he's not the valedictorian of the bomb school."
"If you've handled explosives a lot ... you'd just understand that he just doesn't really understand how all these materials work in concert," says Cavanaugh.
The bomb was made up a pair of five gallon gas cans, three propane tanks, two clocks, M-88 fire crackers, and the nonexplosive fertilizer.
"His training could be anything from being shown a few diagrams to [discussion of] the systems, to months and months and months of training. He did understand somewhat the mechanisms -- he tried to have some improvised detonators in the form of these fire crackers," says Cavanaugh.
And even though the Times Square bomb's clocks look like they're made by Fisher Price (see image above), any timer can do the trick if the device is designed correctly, he says.
According to the May 4 criminal complaint against Shahzad, he said after his arrest "that he had recently received bomb-making training in Waziristan, Pakistan."
Attorney General Eric Holder said Sunday "the Pakistani Taliban was behind the attack. We know that they helped facilitate it." And John Brennan, special assistant to the president for counterterrorism, said on Fox News Sunday that Shahzad had "extensive interaction" with the Pakistani Taliban including for training.
Cavanaugh says that the Pakistani Taliban regularly set off successful car bombs -- in Pakistan -- using both old military ordinance and ammonium nitrate fertilizer. And, the Christian Science Monitor has reported, there are camps in Waziristan where one can learn to make a bomb.
But if Shahzad attended such a camp, he did not take great notes.
Says Cavanaugh: Shahzad's bomb "wasn't going to work on the back end because the explosives weren't explosives. And it wouldn't work on the front end because it was too clever by half."
All that said, Cavanaugh believes that if a fire started in the SUV and the propane tanks blew up, people directly next to the SUV in Times Square could well have been killed.
Here's the FBI's diagram of the device in the 1993 Nissan Pathfinder: