The FBI just held a short press conference, revealing the two current "official suspects." I taped the press conference, but it will take about two hours to upload the video, because the file is quite large. When it has finished uploading, it will be available on youtube HERE. Maybe the press conference will be available somewhere else quicker.
The FBI spokesman said that the two men are the suspects because they went together to the marathon, as can be see in the published surveillance video below, and "suspect 2" with the white baseball cap was seen dropping his rucksack at the site of the second explosion, shortly before it happened. Apart from that, the information which was given during the press conference was rather sketchy. It is apparent that the investigation has only just started, and that there are still many open questions.
The FBI have presented some pretty good pictures of the suspects. All the pictures can be found on the FBI-website here.
Here are some images from the FBI-website:
The FBI warned everyone against taking action on their own, if you come across these people, as they may be dangerous. Instead call: 1-800-CALL-FBI (1-800-225-5324).