They are in fact members of the National Guard's 1st, 13th, and 24th Civil Support Teams: http://communities.washingtontimes.com/neighborhood/freedom-...
They are not mercenaries and there is no evidence to suggest they planted any bombs.
Claims that one of the men was wearing a backpack before the blast and that it was missing after are based on a cropped version of a photograph taken after the blast.
The photograph was taken 1 minute and 57 seconds after the first explosion, and clearly the man on the right can be seen with his backpack: http://i.imgur.com/9vGr70O.jpg
Besides the aftermath of the explosion being clearly visible, the marathon timer shows 4:11:40
The first explosion occurred at 4:09:43 as can be seen here: https://vine.co/v/bFdt5uwg6JZ/embed
Those using a cropped version of the photo, claiming it was taken before the explosion, are either being intentionally dishonest or are unwittingly spreading disinformation.
Check out the Laissez-Faire Journal at LFJournal.com
"The State is a gang of thieves writ large." - Murray Rothbard