The Miranda warning is the law of the land; however, the public safety exception is also the law of the land (so sayeth the U.S. Supreme Court).
"The Miranda rule is not, however, absolute. An exception exists in cases of "public safety". This limited and case-specific exception allows certain unadvised statements (given without Miranda warnings) to be admissible into evidence at trial when they were elicited in circumstances where there was great danger to public safety.
“The public safety exception derives from New York v. Quarles, a case in which the Supreme Court considered the admissibility of a statement elicited by a police officer who apprehended a rape suspect who was thought to be carrying a firearm. The arrest took place in a crowded grocery store. When the officer arrested the suspect, he found an empty shoulder holster, handcuffed the suspect, and asked him where the gun was. The suspect nodded in the direction of the gun (which was near some empty cartons) and said, "The gun is over there...."
https://en.wikipedia.org/wiki/Miranda_warning
In the Quarles case, the question about the location of the gun was asked before any Miranda warning was given; however, the Supreme Court carved out a “public safety” exception to the Miranda rule and the gun was allowed into evidence. Here is a link which does a good job of explaining the public safety exception:
FBI ? The ?Public Safety? Exception to Miranda
Under the law, a suspect may be questioned without being Mirandized in order to neutralize an imminent danger to the public. Whether the questioning of the suspect in the Boston Marathon bombing falls within the exception will be a matter for the courts to decide.
It is my understanding that the questioning would have to be limited solely to prevent further harm to the public and NOT to gather evidence for the purpose of convicting the person. I suspect the questioning will be very limited in scope until such time as the Miranda warning is given. One thing is for certain: no one wants to jeopardize the prosecution of this case and they are not about to commit an obvious violation of the law. If they do heads will roll.