Dershowitz also said statements made by police in Boston seems to contradict the government's reasons for invoking the public safety exception.

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The public safety exception does not set a hard time when questioning must stop and the suspect must be taken into court, but it's generally understood to be about 48 hours.

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The government has invoked the public safety exception, a designation that allows investigators to question the teen without reading him his Miranda rights and without a lawyer present, another Justice Department official, also speaking on condition of anonymity, told CNN.

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"That is highly unusual for a judge to intervene so hastily and make the decision not based on the facts of the interviews and the public safety exception, but what they perceived was happening based on what they saw on television, " he said.

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Under a 1984 Supreme Court ruling, a public-safety exception allows investigators to question suspects for an unspecified period without giving them a Miranda warning.

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Or the government could try to persuade a judge that any confession by Mr. Tsarnaev resulting from such an interrogation could be used as evidence against him under the "public safety" exception to the Miranda rule.

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Executive Director Anthony Romero said the legal exception applies only when there is a continued threat to public safety and is "not an open-ended exception" to the Miranda rule, which guarantees the right to remain silent and the right to an attorney.

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Federal agents at first questioned Tsarnaev without reading him his Miranda rights, under an exception to the rule invoked when authorities believe there is an imminent public safety threat, a Justice Department official said over the weekend.

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But recent amendments to the bill assuaged some of their fears by creating an exception for emergency situations, for example, when there is an imminent threat to public safety or to stop the destruction of evidence.

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The exception is designed to give law-enforcement officials time to determine if there are other threats to public safety.

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