I remember being sent a pic of two men standing side by side with backpacks and 511 tan cargo pants. They two screamed fed or police. But nobody identified them as being part of some security force, of which there was more than two there. My friend sent a link to a pretty good article asking the same questions.
WASHINGTON, April 18, 2013 — Federal and local authorities have yet to comment on the identity of what appears to be a team of operatives photographed at the Boston Marathon bombing site wearing black backpacks, tactical boots and pants and communication equipment. At least one of them wore a hat emblazoned with a skull insignia similar to insignias used by private contractors Craft International and Blackwater USA (renamed Xe Services then Academi), and Navy SEAL teams 3 and 5.
The men were also photographed after the explosion meeting at the finish line behind a black SUV with US Government license plates and satellite communication equipment on the roof.
Infowars.com reports that the photographs were sent to them by Anonymous, along with photos of two possible suspects the FBI asked media not to distribute, but which were later posted by the Drudge Report. The two men were subsequently cleared by the FBI, reports the NY Post.
Shortly after the explosions, several of the operatives ran toward the scene, and one was photographed using what appears to be a radiation detector at the finish line.