[9YearLurker]

They may have known something was up, but they also routinely did such security sweeps for the marathon as well.

[Uncle Chip]

but they also routinely did such security sweeps for the marathon as well.

They probably had the dogs noses dialed onto PEDs rather than explosives.

[eggman]

The real question is whether DHS or the FBI had a information about a potential terrorist attack and didn't share that info with the local police. They may have been watching Tamerlan Tsarnaev but not put the pieces together in time to stop him.