According to this
report, Faisal Shahzad chose to use less effective explosives due to his perceptions of US security measures. Specifically, he believed that if he'd bought the most effective materials, he would have come to the attention of authorities.
Times Square bomber Faisal Shahzad used inferior explosives to avoid detection, New York's police commissioner said Tuesday, helping to explain why an international bomb plot ended up a dud.
. . . .
A key question in the early stages of the investigation had been how a trained terrorist could craft such a poorly made bomb, consisting of weak fireworks, propane tanks and nonexplosive fertilizer.
"He tried to lessen the explosive nature of the fertilizer that was used because he thought he would get a higher profile as he went to buy it," Mr. Kelly told reporters, citing the M-88 firecrackers Mr. Shahzad used, among other elements. "He sort of dumbed that down."
Law enforcement agencies, particularly the Federal Bureau of Investigation, maintain a number of "tripwires" designed to encourage people who sell everyday products that could be used to make explosives to notify agents of any suspicious behavior or purchases. Mr. Shahzad was apparently so worried about the tripwires that he deliberately built a weaker, less effective bomb.
The New York Post reported in Tuesday's editions that investigators have detonated a bomb constructed according to Mr. Shahzad's original planned ingredients, and found that it would have been deadly.
This is similar to the Christmas Pantybomber situation. In both cases the perpetrator used an ineffective device due to the presence of/perceptions of security measures.
I don't think anyone would say that our present security measures (at airports, customs/immigration, or within law enforcement channels) are airtight, and they are certainly not efficiently carried out. But these anecdotal reports at least indicate that we are getting something
for our money/inconvenience. We don't want a police state, but it's possible that relatively small but high-profile anti-terrorism steps can have a useful deterrent effect.
Now, if we can somehow increase the TSA screener ratio of "doing something constructive" : "loitering and chatting with my pals," maybe we could drive down the costs of the present measures somewhat.