Seems like they could check on who bought three or more pressure cookers at one time in the Boston area over the last few months.
They could, but that will take a while as the FBI tries to identify the brand and the stores that sell that brand and then go through all the stores’ records to try to find multiple purchases. If the terrorist was smart, and he probably is, he will have purchased them with cash or one at a time. The store may still have surveillance tapes if they can pinpoint the time of purchase.