Beyond the absence of known links between Mr. Tsarnaev and Al Qaeda, it is also unclear whether the Constitution permits the government to hold citizens arrested on domestic soil as enemy combatants. Though Mr. Graham believes that it would be lawful, other lawmakers disagree. Neither Congress nor the Supreme Court has resolved the question.

During the Bush administration, the Supreme Court upheld the indefinite military detention of Yaser Esam Hamdi, an American citizen who was captured carrying a weapon on the Afghanistan battlefield.

But the court never resolved the case of another American, Jose Padilla, whom the Bush administration held as an enemy combatant for several years after his arrest in Chicago. Two different federal appeals courts disagreed about whether it was lawful to hold someone like Mr. Padilla in indefinite detention without trial, and the Bush administration transferred him back to the civilian court system before the Supreme Court took up the case.

In the Hamdi ruling, Justice Sandra Day O’Connor wrote “there is no bar to this nation’s holding one of its own citizens as an enemy combatant.” But she also wrote the decision was limited to Mr. Hamdi’s “narrow circumstances.” She also said the purpose of wartime detention was to keep captured enemies from returning to fight, adding, “Certainly, we agree that indefinite detention for the purpose of interrogation is not authorized.”

Mr. Graham said the purpose of holding Mr. Tsarnaev as a military detainee would be to question him at length without any lawyer. Though the Obama administration has said it would use a public-safety exception to the Miranda rule to question him for a period without warning him of his rights to remain silent and have a lawyer, Mr. Graham said that would at best gain only a few days before a lawyer intervened.

Mr. Graham also acknowledged that ultimately Mr. Tsarnaev must be transferred back to the civilian criminal justice system for prosecution, because the statute authorizing military commissions — which he helped write — does not apply to United States citizens. Mr. Graham, one of the leading Republican voices opposing the torture of terrorism suspects, emphasized that interrogation as an enemy combatant would not mean inflicting suffering on Mr. Tsarnaev in order to make him talk, and that anything he said in military detention should not be used as courtroom evidence.

But Mr. Graham rejected the view that the Federal Bureau of Investigation might be equally or more effective than intelligence officials at persuading Mr. Tsarnaev to provide information, even with a defense lawyer at his side, since the law enforcement officials would have the leverage of being able to float more favorable treatment in exchange for cooperation as part of a potential plea bargain. He said interrogators needed to build a rapport with a prisoner, suggesting that any presence of a lawyer would disrupt that dynamic.

“That is, to me, the dumbest way to induce someone to talk,” he said of such tools and tactics of law enforcement officials. “I want intelligence officials trained in the intelligence process to have a chance to talk to him, without a lawyer.”