For the best current information, look for press conferences by the Boston PD and FBI. The 24-hour news channels have to fill airtime, even when there are no new developments, so they run with what they have. And if they are careful to weasel-word their announcements, some people with CNN or Fox on in the background will miss the weasel words and treat the rumors like truth.
I did hear from a witness who reported that the smoke from the explosion smelled of sulfur, which tends to point toward a blackpowder explosive (and thus tends to indicate a lone amateur or small group - well-organized groups would use more powerful explosives); however, please keep in mind that this is unconfirmed, and cannot be fully trusted until forensics have been completed. (For instance, the witness may have expected to smell sulfur, and filled that little detail in from imagination; the detail would then be a "concrete" part of the memory, whether or not it was objectively real. That's one of the reasons why eyewitness testimony is considered unreliable.)
_________________
Sodium is a metal that reacts explosively when exposed to water. Chlorine is a gas that'll kill you dead in moments. Together they make my fries taste good.