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“The ‘public-safety exception’ is precisely that an emergency exception,” said Steven Benjamin, president of the National Association of Criminal Defense Lawyers. “[Tsarnaev], a U.S. citizen, arrested on U.S. soil, has been in custody for nearly three days and the government will have a heavy burden to show that any further questioning without a reading of his Miranda rights is justified.

With Tsarnaev in a hospital recovering from a severe wound, it’s unclear whether any interrogations have taken place and how much law enforcement and intelligence officials have gleaned. The White House declined to publicly discuss details of any interrogations or confirm they took place at all.

The stakes are high for the Obama administration, the Justice Department and the FBI, which is already under scrutiny for failing to closely monitor Tamerlan Tsarnaev, the elder brother who died during a confrontation with police after Russia’s FSB security service warned the U.S. intelligence authorities about his ties to radical Islam in 2011.

Mr. Graham on Monday called for congressional hearings to look into how Tsarnaev slipped through the cracks. He said FBI officials told him Sunday that they followed up on the tip, but did not find any terrorist activity after interviews with him, his family and officials at his school.

The FBI asked Russian officials for more information on Tsarnaev but never received a response, he said. Mr. Graham said that the FBI was unaware that he traveled to Russia for six months in 2011 because his name was misspelled by the airlines.

Seth McLaughlin and Sean Lengell contributed to this report.