FINAL UPDATES FOR NOW: 3 dead; 140+ injured; Video of blasts; Obama vows 'full weight of justice'; Bombs said placed in pressure cookers; No known motive or suspect...
By Brad Friedman on 4/15/2013, 12:38pm PT  

[UPDATED MANY TIMES AT BOTTOM OF ARTICLE, NOW INCLUDING FINAL WRAP (FOR NOW) ON THE DAY AFTER.]

From AP, via National Memo...

BOSTON (AP) — Two explosions at the finish line of the Boston Marathon have resulted in injuries.

Bloody spectators were being carried Monday to the medical tent that had been set up to care for fatigued runners. Police wove through competitors as they ran back toward the course.

“There are a lot of people down,” said one man, whose bib No. 17528 identified him as Frank Deruyter of North Carolina. He was not injured, but marathon workers were carrying one woman, who did not appear to be a runner, to the medical area as blood gushed from her leg. A Boston police officer was wheeled from the course with a leg injury that was bleeding.

About three hours after the winners crossed the line, there was a loud explosion on the north side of Boylston Street, just before the photo bridge that marks the finish line. Another explosion could be heard a few seconds later.

More photos at ThinkProgress...

The blasts reportedly occurred near the Marathon headquarters in a nearby hotel. The headquarters has been locked down. Additionally, NBC News is reporting:

Janet Wu, a reporter for NBC affiliate WHDH, told NBC News that she heard two loud explosions. Jackie Bruno, a reporter for New England Cable News, said on Twitter that she saw people’s legs blown off.

“Runners were coming in and saw unspeakable horror,” she said.

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MANY UPDATES THROUGHOUT THE DAY, AND A WRAP UP ON THE DAY AFTER, ALL NOW FOLLOW BELOW...

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UPDATE 12:49pm PT: Here is video captured of one of the explosions...

Live NBC coverage is streaming here...

UPDATE 12:55pm PT: NBC is now reporting a large fire at JFK Library nearby...

UPDATE 1:07pm PT: Three separate law enforcement officials tell NBC that "one small improvised explosive device" the cause for one of the blasts...

UPDATE 1:10pm PT: AP: Boston Police report 2 dead, 23 injured...

UPDATE 1:23pm PT: AP: "Intelligence official: 2 more explosive devices found at Boston Marathon; being dismantled"

UPDATE 1:34pm PT: Photos from Boston Globe's David L. Ryan and John Tlumacki [click each for larger images]...

Also, 28 now reported injured...

UPDATE 1:54pm PT: Dorchester Reporter reports JFK Library spokesman says fire not related to the explosions at the Boston Marathon. It was "inside the building's mechanical room...not caused by a device". Boston Police Commissioner, however, just described what happened at the JFK Library as a "third explosion", but adds he is not aware of any injuries there. So what happened at the JFK Library would still seem to be unclear.

New video now of the second explosion at the Marathon...

Finally, for now, Boston Globe just tweeted that the number of injured has risen to 64...

UPDATE 2:10pm PT: AP: "A law enforcement official says cellphone service has been shut down in the Boston area to prevent any potential remote detonations of explosives."

UPDATE 2:20pm PT: Lengthier video of first explosion, via Boston Globe, including cops running towards explosion afterwards, servicemen arriving to help, and bloodied victims on ground...

UPDATE 2:59pm PT: Boston Police Commissioner now says that the JFK Library incident was an "incendiary device or a fire" and that "the two events are not related," contradicting his earlier remarks that there had been an explosion there. He also says, contrary to misreporting from Rupert Murdoch's NY Post, that "there is no suspect in custody".

Also, President Obama is set to give televised remarks at 3:10pm PT.

UPDATE 3:34pm PT: The President just finished his brief remarks. Read them in full here. Here's a key paragraph...

We still do not know who did this or why and people shouldn't jump to conclusions before we have all the facts. But, make no mistake, we will get to the bottom of this and we will find out who did this. We will find out why they did this. Any responsible individuals, any responsible groups will feel the full weight of justice.

UPDATE 3:51pm PT: CNN now reporting more than 110 injured, many in critical condition. One of the confirmed dead is an 8-year old boy.

UPDATE 5:58pm PT: Boston Police Commissioner Ed Davis updates the death toll to "at least 3". Davis also attempts to clear up continuing confusion about a "suspect" reported by several news outlets now as being held at a local hospital:

"I want to stress one thing," Davis said. "There is no suspect at Brigham and Women's Hospital. There are people that we are talking to but there is no suspect at Brigham and Women's Hospital as has been widely reported in the press. I would like to fix that right now."

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UPDATE 4/16/2013, 10:30am PT: We strived, as usual, to only offer well-sourced reports in our coverage over the day yesterday. Over all, we did fairly well on that score. But just to wrap up our live blog coverage here before we move on to new items, here's a quick review of what we know the next day, as well-summarized by Josh Marshall this morning...

The central point is that there is still no clear explanation of who was behind the attack or why it happened. It now appears clear that, despite yesterday's reports, there were no additional explosive devices besides the two that exploded near the finish line at the marathon, the two we knew about from the first moment. The initial report of a further bombing at JFK Library seems to have been ruled out definitively as have various reports of other unexploded devices found in the vicinity of the attack.

Three people are confirmed dead in the attack, one of whom was an 8 year old boy. More than 140 were wounded.
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All the signs from law enforcement suggest they do not yet have a clear theory of the crime.

At this point, no individual or group has claimed responsibility for the attack.

Irresponsible reports yesterday of a Saudi national in custody and 12 killed at the bombing have all now been debunked (though the media which offered those reports have not necessarily apologized.)

Over the past hour, AP has reported that, according to an unnamed "person briefed on the investigation," the two bombs "were fashioned out of pressure cookers and packed with shards of metal, nails and ball bearings." They are said to have been placed into backpacks.

Police and federal agents announced they are asking "for any video, audio and photos taken by marathon spectators, even images that people might not think are significant," to help aid in the investigation.

Finally, here is a photo of the 8-year old boy Martin Richard who was among the three killed yesterday. It was taken during a May 2012 "peace walk" he took part in with his school...