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BOSTON -- The arrest of Dzhokhar Tsarnaev ended the manhunt for the BostonMarathon bombers, but it set in motion an equally intense phase of thecase that will begin with the grilling of the man who --for now at least-- is the only surviving suspect.

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Bombing suspect captured in Watertown

An indication of the complex investigation ahead came Fridaynight, when an Obama administration official told NBC News that Tsarnaevwould not be given a Miranda warning when he is physically able to beinterrogated after receiving medical treatment.

Instead, theofficial said, the government will invoke a legal rule known as the"public safety exception," which will enable investigators to questionTsarnaev without first advising him of his right to remain silent and tobe afforded legal counsel.

The exemption can be invoked when information is needed to protectpublic safety. In this instance, the government believes it's vital tofind out if Tsarnaev planted any other explosives before his capture orwhether others might have plotted with him to do so, said the official,speaking on condition of anonymity.

Tsarnaev's older brother, Tamerlan, was killed in a shootout withpolice early Friday, and it was not clear until late Friday thatauthorities would be able to question their remaining prime suspect.

Untilshortly before his capture around 8:45 p.m. ET, the wounded andbleeding Tsarnaev exchanged gunfire with authorities in Watertown,Mass., while sheltering in a plastic-wrapped pleasure boat.

Officers on the scene and the brass in the command center were both clearly elated by the outcome.

"Wealways want to take someone alive so we can find out what happened,"Boston Police Commissioner Ed Davis said at a media briefing an hourlater, "and we can hold them to justice."

High Value Detainee Interrogation Group

Therule waiving the Miranda warning does not set a precise limit on howlong a suspect can be interrogated before being advised of his rights,but it likely buys authorities no more than 48 hours.

During that timeTsarnaev, 19, will be questioned by a federal government team called theHigh Value Detainee Interrogation Group, consisting of officials of theFBI, CIA and Defense Department. Though he will not have a lawyerpresent, any statements he makes during the questioning will beadmissible in court.

Among the questions investigators are certainto focus on is whether he and his brother had help in plotting orcarrying out the terrorist attack at the finish line of the marathon.The dual blasts from pressure cookers packed with explosives andshrapnel killed three people and injured 176.

That question tookon more urgency when police in New Bedford, Mass., south of Boston,announced Friday evening that three people there had been taken intocustody as part of the bombing investigation.

In addition topossible co-conspirators in the U.S., the interrogators also will wantto know whether the brothers, both ethnic Chechens, received anyassistance from overseas.

Travel records obtained by NBC New Yorkshowed that Tamerlan Tsarnaev left the country for six months in 2012,flying to Moscow on Jan. 12 and returning on July 17. Where he went andwhat he did after his arrival in Russia could expand what so far hasbeen a domestic manhunt into a global one.

Enemy combatant?

Suspicionsthat the elder brother could have received terrorist training orsupport abroad were heightened Friday, when an official familiar withthe matter told NBC News that a foreign government had expressed concernin 2011 that Tamerlan Tsarnaev could have ties to terrorism. Theofficial said the FBI investigated, but found no such links and reportedthe findings back to the foreign government.

Even if authoritiesdetermine that the Tsarnaevs received support from an overseas terroristorganization, the Obama administration official said the governmentwill not seek to declare him an enemy combatant and try him before amilitary commission, as it has done with senior al Qaeda officialscaptured overseas and imprisoned at the Guantanamo Bay detention camp.Administration officials see that scenario as a non-starter, theofficial said, particularly given the fact that Dzhokhar Tsarnaev is anAmerican citizen, naturalized last September.

Republican Sens. Lindsey Graham of South Carolina and John McCain ofArizona issued a statement late Friday urging that the administrationhold Tsarnaev as an enemy combatant.

"It is absolutely vital the suspect be questioned for intelligencegathering purposes. We need to know about any possible future attackswhich could take additional American lives," said the statement, posted on Graham's Facebook. "The least of our worries is a criminal trial which will likely be held years from now."

Mass of evidence

At the same timethey are seeking to uncover the bombing suspects' motives and determinewhether they had a support network, investigators will continue tocollect and analyze vast amounts of forensic evidence from crime scenesstretching across three cities.

In addition to processing evidence from the bombings, FBI technicianswill analyze hundreds of hours of video camera recordings from privateand public surveillance and traffic cameras as they attempt to trace thebrothers' movements - both after the attack and before it.

Investigatorsalso will obtain and assess phone records, seeing who the brothers werein contact with in the weeks and months leading up to the attacks.

Onlywhen they have scrutinized every bit of data, and explored every lead,will they turn over the mountain of evidence they have assembled toprosecutors. It will be up to them to decide what charges the youngerTsarnaev should face and whether to seek the federal death penalty in astate where life in prison is the maximum sentence that can be imposed.

Butdespite such a massive expenditure of time and technological know-how,they may never answer the most haunting question surrounding the case,as President Barack Obama noted.

"Why," he asked during a briefstatement on Dzhokhar Tsarnaev's arrest late Friday, "did young men whogrew up and studied here as part of our communities and country resortto such violence?"

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