I’m sure they asked their handler what to do after the bombs went off. Their handler probably said something like, “Don’t worry, guys. They will never suspect you two being involved in it since you look like the average, college-age, American joes. Just go back to school, hang out, relax, party, blend in, and have a good time. Those stupid, American infidels will never figure out its you two!”
And the brothers believed it.
“And the brothers believed it.”
And they were almost right!
I think the cell leaders thought these 2 would be immediately caught and severed all links before the bombing. Phase II was to wait for the police to find them, and go out in a blaze of glory. When the cops never came looking for them (thanks to holding onto the photos of the real perps) they went looking for the cops to get the same result.
Also possible is that they were trying to reach a compatriot but never made it.