Even if no one ever tells him so, Dzhokhar Tsarnaev has the right to remain silent. And even if he never utters another word, there is probably more than enough evidence to convict him.

So all the concern about whether authorities gave him a Miranda warning, informing him of his right to remain silent and to have a lawyer, is sort of beside the point. Sort of ― but not entirely.

Tsarnaev is a suspected criminal, not an enemy combatant, as the Justice Department properly acknowledged by charging him in federal court with using a weapon of mass destruction. Depending on what his and his brother’s motives turn out to be, he may also be a terrorist, which is why the FBI has properly decided not to read him his rights yet.

The main purpose of the Miranda warning is to protect the rights of the accused. Practically speaking, however, it helps protect law enforcement at least as much: Without it, any information authorities get from a suspect is inadmissible in court. They are always free to ask whatever they want. They are not always free to present it as evidence.

There are a few exceptions to this rule, and the relevant one in the Boston Marathon case is what is known as the “public-safety exception.” If police have reason to believe that there is a ticking time bomb, say, or if a fleeing suspect tosses a loaded gun into the soap aisle at the supermarket, then authorities can question him without reading him his rights and still use the evidence against him. Three years ago, the FBI expanded this exemption. Now ― at least according to the FBI ― it can question a terrorist “to collect valuable and timely intelligence not related to any immediate threat.”

No one knows exactly why the FBI has invoked the exception now, only that it has. Nor has the FBI’s expansive reading been tested in court. A more specific explanation of its use in this case and a more general opinion of the legality of the new reading are essential.

Regardless of its extent ― and the exemption shouldn’t be open-ended ― it won’t much affect the fate of 19-year-old Dzhokhar Tsarnaev. It appears the Justice Department can easily build a criminal case based on the voluminous video, forensic and eyewitness evidence collected in the last week. Each hour brings more news about the what, where and when: the bombs in the backpacks, the fatal shooting of a campus police officer, the car chase through the suburban streets, the hideout and eventual capture in a backyard boat.

What remains are the why and the how. “Why did young men who grew up and studied here, as part of our communities and our country, resort to such violence?” President Barack Obama asked last week. “How did they plan and carry out these attacks, and did they receive any help?”

Not just the investigators deserve to know the answers to those questions. That’s why legal rules such as the Miranda warning can be so important, and should be suspended only sparingly: They increase the chances the public will get a full accounting of the crime. Otherwise, anything Tsarnaev says may never find its way into trial. (Tsarnaev, who remains in serious condition and unable to speak, is reportedly responding to some questions in writing.)

The criminal-justice system isn’t always good at providing such answers. Most cases, of course, never reach trial. Some trials, especially ones involving terrorists, are closed. Some become media spectacles. And some questions, especially ones about a disaffected 19-year-old’s motivations, may be essentially unanswerable.

By all means, interrogate Dzhokhar Tsarnaev about his political motivations and terrorist connections, without a lawyer present if national security demands. Eventually he will get, and deserves, a lawyer. And the U.S. will get, and deserves, a public trial. It may not be perfect, but it is America’s first, best shot at getting answers

(Bloomberg)