" Dzhokhar and Tamerlan Tsarnaev were identified by an examination of footage from numerous private security cameras that were recording the crowd in downtown Boston during the Marathon. Imagine if they could have dispatched their bombs in the trunk of a car that they were never in themselves? Catching them might have been an order of magnitude more difficult than it was."
Let's see. Car...registration, license plates...probably fingerprints, and random DNA from hair. Yeah, that will be a "magnitude more difficult", even if it's a rental.