Well, there's this video...
....the tweets from the Boston Globe (shown in the OP)
...this from a timeline:
Around 3:55, the bomb squad set off a controlled explosion opposite the library.
An official tells the AP that additional devices were located, in addition to the two that exploded. It's not known if this includes the one detonated by the bomb squad.
Possibly related, at 4:27 the scanner reported a confirmed explosion at the JFK library, with no injuries reported. (This explosion must be the one mentioned in the Globe tweets).
http://deadspin.com/explosions-reported-at-the-boston-marathon-473008941
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So what we've got are two bombs going off by persons unknown and at least one set off by the authorities, which was pre-planned as part of a drill, apparently.
This is so weird. Who does a live-fire drill with bombs in the middle of a city when a marathon is being run? Who would have the balls to plant bombs right in the thick of a bunch of cops/national guard with bomb sniffing dogs? Why would they go ahead with the pre-planned explosions after the bombs went off?
Who is this guy?