6:38 AM PT: The briefing will begin in about two minutes.
6:40 AM PT: The briefing includes Boston's mayor, Massachusetts' governor, and the FBI's Special Agent In Charge. Governor Patrick is starting it now.
6:42 AM PT: Patrick: There were "two and only two explosive devices."
6:44 AM PT: Patrick says more than 150 were wounded. He thanks first responders for their response and service. Patrick turns the briefing over to Boston mayor Tom Menino.
6:47 AM PT: Senator Elizabeth Warren speaks on behalf of the state's congressional delegation.
6:50 AM PT: Rick DesLauriers, the FBI Special Agent in Charge for Boston takes the podium. "Our mission is clear: to bring to justice those responsible for the Marathon bombing." The investigation is ongoing, he says. There are no other active threats, he says. He asks the public to help the investigation and encourages more assistance. Tips should call 1-800-CALL-FBI, he says.
6:52 AM PT: The ATF Special Agent in Charge reiterates Patrick's statement that there were only two devices. He specifically asks for any video or photographic recordings and urges people to call the FBI tipline.
6:57 AM PT: Boston Police Commissioner Ed Davis says the crime scene has been collapsed to about 12 blocks and expects it to remain for about 2 days. He reiterates the ATF SAIC's request for any videos or photos. And now the state police commissioner says the public should expect to see an increased police presence in coming days, but that it's not a response to a specific threat, instead it's for "the comfort of the public." He reiterates the request for video and photographic evidence that had been previously mentioned.
6:59 AM PT: So far at least, nothing as far as information regarding any sort of leads or anything like that. The briefing is primarily focused on process and encouraging the public and witnesses to assist and cooperate with the investigation. Patrick will open it up to questions—maybe we'll get some new info here.
7:01 AM PT: DesLauriers is doing a lot of not commenting right now.
7:05 AM PT: Commissioner Davis said the Boston police have been collecting video from local business surveillance. Davis also just referred to the Boston as facing a "threat" which seems a little different than what DesLauriers said earlier about there being no active threats. Maybe it's just imprecise language.
7:08 AM PT: Davis says there were 176 injuries, 17 critical, including 3 deaths.
7:10 AM PT: The briefing has concluded.