Not sure which scanner you’re listening to but that is being denied by the FBI at the moment.
http://tunein.com/radio/Boston-Police-Fire-and-EMS-Scanner-s146109/
The reports which contradict the FBI are on WHDH Ch7 and WCVB Ch 5 Boston.
http://www1.whdh.com/video/7newslive http://livewire.wcvb.com/Event/117th_Running_of_Boston_Marathon