Major cities are ramping up security in response to the bombing at the Boston Marathon here in New York critical response teams were deployed around the city. At hotels and landmarks. In LA they're... See More
Major cities are ramping up security in response to the bombing at the Boston Marathon here in New York critical response teams were deployed around the city. At hotels and landmarks. In LA they're dispatching bomb -- bomb sniffing dogs. At airports and tonight's LA Dodgers games and also in London British police are reviewing security plans. For the London marathon on Sunday. So what have we learned about the investigation. The latest on who is behind this and the nature of these bombs let's go to ABC's senior justice correspondent. Who has been working this story all day Pierre Thomas Pierre. -- but we're being told by sources -- that they believe that too. Portable devices were left in place at the scene and then detonated. Law enforcement officials right now as we speak are at the scene they are looking for pieces of this that the device. It were to begin to put them back together to give a sense of who. Might be involved they're also looking for surveillance tape any surveillance tape that would give them an idea of who might have done -- Also we're told in addition to looking at potential domestic suspects they're also looking at people trying to lead the Boston area airport. Everything is being looked at Diane this is tied -- highest hyper. Priority of the federal government can compare -- -- the video explosion we're gonna slow it down. Look again at this video totally for rent -- what investigators. Think they can determine. Just from what -- -- singing. Well I spoke with -- sources who were watching. On our air and witnessing that particular videotape and they say it confirms what they think if you look at it very closely it seems to come from a fairly confined small area. Again that was one of the reasons why they thought that this was some sort of -- left behind in the midst of that crowd to harm everyone in the immediate proximity. And -- persons of interest tonight any leads at all. Again all of the above -- one source told -- there will look at domestic groups and look at international groups until they can get a shred of evidence that points than any particular direction. Everything is on the -- And again as has so much that they are asking that everyone who has any video at all send it to them. And send them any leads Pierre Thomas who keep working throughout the night I know.
This transcript has been automatically generated and may not be 100% accurate.