[iontheball]

Before anyone was even caught they somehow knew that these two guys were the only people involved.
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While they feigned ignorance on the black cap bomber, they finally admitted to knowing him from the Russian heads-up and their 2011 interview. Obviously, red flags should have gone up at that time and especially when he traveled to Russia. They should have tracked where he went after arriving in Moscow or asked Putin to have him tracked. Then he returns after 6 months and apparently no FBI followup. However, when they saw him in the marathon crowd, they knew they immediately knew they had screwed up.

[Fully Awake DAV]

Last line of your post
“However, when they saw him in the marathon crowd, they knew they immediately knew they had screwed up.”

Could be some agency had actually provided,what they thought were, inert materials for the explosive devices, therefore they were monitoring the individuals in order to sweep in with bomb detection devices to “make the bust”. This could have been a “look at what we did” or a “look we need more funding” situations.