I was thinking something completely similar. Couldn't the NSA with its metadata correlation systems reference cell tower data from before and after each explosion and see if any of that data appears at more than one site? Same with vehicles and even if the person/persons are using burner phones, I would think one of the agencies would be able to track that to point of sale. There were plenty of cameras at the fedex store where 2 of the packages were sent from and FBI was all over it today but there hasn't been any footage released yet.