You may have heard that on Thursday the new head of the Boston FBI office, the Boston Police Commissioner, and an official of the Massachusetts State Police issued a joint statement that said in part:

Members of the Joint Terrorism Task Force did not know [the Boston marathon bombers'] identities until shortly after Tamerlan Tsarnaev’s death when they fingerprinted his corpse. Nor did the Joint Terrorism Task Force have the Tsarnaevs under surveillance at any time after the Assessment of Tamerlan Tsarnaev was closed in 2011. The Joint Terrorism Task Force was at M.I.T., located in Cambridge, MA, on April 18, 2013, on a matter unrelated to the Tsarnaev brothers. Additionally, the Tsarnaev brothers were never sources for the FBI nor did the FBI attempt to recruit them as sources.

(Statement quoted in full at the end of http://www.myfoxboston.com/story/23722778/tough-questions-for-fbi-over-marathon-bombing-response. April 18 was three days after the bombing, and the night Officer Collier and Tamerlan Tsarnaev were killed.)

As noted in the linked article, the statement was issued in response to questions from Senator Chuck Grassley (R-IA) in his capacity as a member of the Senate Judiciary Committee, in a letter to the new FBI Director James Comey. Grassley had heard that the Cambridge area was under surveillance, and seems to have wondered if the Tsarnaevs somehow slipped through it, resulting in the death of Officer Collier (i.e., under the assumption that they were guilty of that).

But more specifically, the linked article says that

FOX Undercover has learned one explanation why there were at least some FBI surveillance teams in Cambridge: several MIT students were being looked at as suspects.

Well. from a point of view that does not assume the Tsarnaevs’ guilt a priori I have different questions than does Sen. Grassley. As far as one can tell, at the time of the April 18 FBI’s press conference identifying certain pictures as those of the suspects, it had for evidence exactly one surveillance video showing Dzhokhar Tsarnaev putting down his backpack. (It may or may not have also had by then the eyewitness to Tamerlan Tsarnaev putting down his, I’m not sure.) That is all. If it had, say, an eyewitness to Dzhokhar putting down his backpack the government would have said so in a recent court motion where it listed (pg. 11) the “most compelling” evidence against him supplied to the defense in discovery.

What I want to know is: why were one or two images of the Tsarnaevs putting down their backpacks, among all the other spectators who must have done that during the marathon, of greater weight than whatever led the Boston FBI to surveil those MIT students and whatever the surveillance turned up?

Update 10/20/13 11:45 AM Eastern: The JTTF letter quoted above says they did not connect the dots until they fingerprinted the corpse. But as TarheelDem points out @ comment #7 below (and see my response @ 8), fingerprints had nothing to do with what they claimed to know about the bombers at that point; they only had pictures. Sounds like some signals are crossed here.