Subsequent terror attacks against the US were not thwarted by high technology, even with all of the high technology and invasiveness that has followed. Those that were detected in advance were done so because someone in the public reported it, those that were tried and failed did so because of problems of the terrorists' makings, and those that succeeded (like Boston) happened in part because high technology failed to do its job and find those who would do us ill.
The trouble is that the TLAs get a lot of public reports and a lot of diplomatic tips. After every 'successful' event, from Fort Hood to Eric Snowden, it turns out there were warnings and signs that got overlooked or ignored. Now, it's possible that these warnings only exist because the individuals were actually dangerous. It's also possible that they're overlooked because they're so ubiquitous as to be useless. If you think back through your own 'file,' are you going to find anti-establishment outbursts in high school? Heated reactions to an idiotic co-worker? Times you've said congress "should just die in a fire?"
If you've got 10,000,000 tips, and 4 of them actually identify someone on the verge of violence, then those tips are pretty useless. If you're a human, you ignore them. If you ignore them, then after-the-fact, some reporter can plant a bright red Incompetent flag on your face. The CYA strategy is to look for ways to cross-reference those 10,000,000 data to find which ones identify the same person and try to whittle down to a manageable number. It's still going to be useless, but at least you'll be able to blame the technology: the algorithm didn't get enough data.
The real problem is that people have built a narrative of War around these terrorist organizations, aggrandizing them into cohesive structures with homogeneous goals and internal coordination that doesn't exist. We should treat the Boston Marathon attack the same way we treated Columbine: a criminal outburst by individuals with perceived persecution trying to make a big statement of outrage. The attackers may have been encouraged by 3rd parties, but they are fundamentally individual decisions and acts.