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7 biggest unanswered questions over Boston Marathon bombings

As prosecutors prepare to charge 19-year-old Dzhokhar Tsarnaev in the deadly twin bomb blasts at the Boston Marathon, investigators are delving into numerous unanswered questions surrounding the attack and the brothers believed to have carried it out.

Officials say Tsarnaev is unable to speak due to a possibly self-inflicted throat wound, but he is responding to investigators’ questions in writing during brief interview sessions being conducted by a special interrogation team. And piecing together the facts will rely very heavily on him, since his older brother, Tamerlan, was killed early Friday during a gunfight with police in Watertown, Mass.

As we wait for word on what the suspect is saying, here are a few of the critical unanswered questions investigators are certain to put to him:

Were more attacks planned?

The most important question for investigators to answer is whether Tsarnaev and his brother had hoped to carry out more attacks and, if so, had they taken any steps toward planning them.

The brothers clearly had the means to cause more destruction: authorities say they have recovered what adds up to an arsenal of deadly weapons from various crime scenes, including homemade bombs, grenades, guns and hundreds of rounds of ammunition.

Police Commissioner Ed Davis said the stockpile suggests the marathon attack was intended to be just the first in a series.

"We have reason to believe, based upon the evidence that was found at that scene -- the explosions, the explosive ordnance that was unexploded and the firepower that they had -- that they were going to attack other individuals. That's my belief at this point." Davis said Sunday on CBS' "Face the Nation.

Did terrorist groups overseas aid the attack?

Investigators are focused on a six-month trip that Tamerlan Tsarnaev took in January 2012, traveling first to Moscow, then to Dagestan and Chechnya, according to his parents.

In an interview Sunday with The Associated Press, Anzor and Zubeidat Tsarnaev said he spent his time there sleeping and visiting with relatives -- not consorting with Islamic terrorists.

But some U.S. officials suggest that Tamerlan was not behaving like a typical vacationer.

Rep. Mike Rogers, R-Mich., chairman of the House Intelligence Committee, said Sunday on “Meet the Press” that the elder brother may have traveled under an alias.

Suspicions that Tamerlan, who reportedly turned to more fundamental Islamic beliefs in recent years, could have received terrorist training or support abroad were heightened Friday, when an official familiar with the matter told NBC News that a foreign government (later said to be Russia) had expressed concern in 2011 that he could have ties to terrorism. The official said the FBI investigated, but found no such links and reported the findings back to the foreign government.

Were Islamist militant groups involved?

It remains unclear if Tamerlan had contact with Islamist militant groups in the restive and predominantly Muslim Caucasus region, which have been blamed for a variety of terrorist acts within Russia. 

On Sunday a group leading an Islamist insurgency against Russia told Reuters it was not at war with the United States and distanced itself from the Boston bombings.

"We are fighting with Russia, which is responsible not only for the occupation of the Caucasus but for monstrous crimes against Muslims," said a statement from Caucasus Emirate militants operating in Dagestan reported by Reuters.

Why target the Boston Marathon?

The motive behind the bombing at the Boston Marathon is the biggest mystery: Why would the brothers do this?

While the Tsarnaevs had surely had some issues adjusting to life in America, friends say they appear to have settled in fairly well since they arrived a decade ago.

Tamerlan was an accomplished boxer and even had sights on the U.S. Olympic team at one point.

Dzhokhar, who became a naturalized citizen last year, is almost universally described as a well-adjusted and smart teen.

While both of the brothers followed Islam, friends say they didn’t espouse violence or appear to be radicalized.

At some point Tamerlan Tsarnaev apparently adopted more fundamental Islamic beliefs, but no accounts have yet emerged to suggest he suddenly saw violence as an acceptable expression of his faith.

Were the bombs homemade, or were others involved?

Federal officials examining the bomb residue collected in Boston have described the bombs – pressure-cookers packed with explosives and shrapnel – as relatively simple in their basic design, but with  sophisticated components, including what appeared to be an electronic detonation mechanism capable of being triggered by cellular phones.

Al Qaeda-linked groups and others have published instructions for making such bombs on the Internet, but it remains unclear whether those blueprints would have provided adequate instruction.

Authorities also say it is unlikely that the pressure-cooker bombs could have been successfully detonated without some practice runs, but so far they have no reports of such trials.

Investigators also say they are still trying to determine where the suspects got their guns.

Why didn’t the brothers flee after the bombings?

Investigators continue to probe Dzhokhar's behavior after he returned to the campus of the University of Massachusetts at Dartmouth on Tuesday and resumed the life of a student after allegedly dropping a bomb-filled knapsack into a crowd watching the marathon.

“There is evidence of some frankly kind of normal student behavior in those ensuing days, which when you consider the enormity of what he was responsible for certainly raises a lot of questions in my mind and as I say more to the point in the minds of law enforcement as well,” Massachusetts Gov. Deval Patrick said Sunday on “Meet the Press.” “Those are the kinds of leads that still have to be pursued and run to ground.”

Tamerlan also was said to have returned to his Cambridge home before a fatal chase and gunbattle with police in Watertown.

The suspects' actions suggest that they did not expect to attract the attention of authorities, at least in the immediate aftermath of the attack.

How did Tamerlan Tsarnaev die?

Tamerlan Tsarnaev was killed during a ferocious gunbattle early Friday with Watertown, Mass.,  police, but officials have not yet determined how he died.

After allegedly shooting to death MIT police officer Sean Collier, 26, in Cambridge late Thursday night, and then carjacking an SUV nearby, the Tsarnaevs were tracked by “pinging” the carjack victim’s cellphone, which had been left in the vehicle, Watertown Police Chief Ed Deveau told the Boston Globe.

When police caught up with the pair later that night in Watertown, the brothers, then in separate vehicles, jumped out to engage officers in a gunbattle.

“Quickly we had six Watertown police officers and two bad guys in a gunfight,” Deveau told the Globe, saying at least 200 shots were fired. The brothers tossed what appeared to be a pressure cooker bomb at the officers and there was a large explosion. “Crude grenades” were also tossed at officers, with three exploding, Deveau said.

Minutes later, Tamerlan walked toward officers firing, but then appeared to run out of bullets, Deveau said. Seizing on the opportunity, officers tackled him.

As they struggled to handcuff the suspect, the stolen Mercedes SUV came roaring at the officers, who scattered. According to Deveau, the younger brother plowed over Tamerlan, who was then briefly dragged under the SUV.

Dzhokar left the SUV on a nearby street and fled on foot, according to Deveau.

While the account suggests Tamerlan died from injuries suffered when he was run over, an autopsy report on his death has not been released.

NBC News’ Pete Williams, Tom Winter and Michael Isikoff contributed to this report.