Out of the tens of thousands of people they investigate annually, and the priorities they are given as potential threats to the US, I'm not at all surprised the FBI couldn't immediately ID TT from a memory of interviewing him years before. It's unrealistic to think that such small time, twice-removed from more pressing threat, suspicious people would ping at high levels. Sometimes little, nothing, low-level, unthreatening people make kitchen bombs and kill folks. Or take their mother's gun and kill folks.
I was listening to a radio interview about how many "high threats" we watch every day in this country. The ones we prioritize as most likely to be a threat to security. There are loads of big threats! To think that we would employ via tax dollars enough people to track medium threats - nevermind the small threat that TT was before the marathon - we'd have to employ 100x the number of "watchers" to track every blip on the security screen. It's simply not realistic. Assuming TT was in a cell of 2, the other being a family member, without "chatter" or threats or warnings, without large weapons, without connections to larger known groups - even the friends of these guys didn't suspect, so it's unlikely that there was much warning observable to anyone else.
Jeez, we're in the midst of all kinds of financial cutbacks. I'm wondering what the figure would be for employing thousands more security - maybe tens of thousands. Where would the money come from?
