Why did this happen? Authorities now asking what motivated Boston bombings

State Police Col. Timothy Alben, accompanied by Massachusetts Governor Deval Patrick, speaks during a news conference, after the arrest of a suspect of the Boston Marathon bombings in Watertown, Mass., Friday, April 19, 2013. (AP / Matt Rourke)

With the prime suspect in the Boston Marathon bombings under guard in hospital with serious wounds and his older brother and accomplice dead, the United States now tries to determine what would prompt two Russian-born Chechens to turn so violently against their adopted nation.

President Barack Obama, expressing the same sentiments that landed Liberal leader Justin Trudeau in hot water earlier this week north of the border, has pledged to direct investigators to determine why Tamerlan and Dzhokar Tsarnaev may have travelled down the path to domestic terrorism.

"There are still many unanswered questions," Obama said in a late-night statement at the White House on Friday, shortly after police captured a bloodied Dzhokar, 19, hiding in a boat in a backyard in suburban Watertown after one of the biggest manhunts in American history.

"Among them, why did young men who grew up and studied here, as part of our communities and our country, resort to such violence? How did they plan and carry out these attacks, and did they receive any help? The families of those killed so senselessly deserve answers."

Obama also urged Americans not to "rush to judgment."

Earlier this week, Prime Minister Stephen Harper pilloried Trudeau for saying in a CBC interview that, if prime minister himself, he would "look at root causes" in the aftermath of a similar attack in Canada.

"We don't know if it was terrorism, or a single crazy, or a domestic issue or a foreign issue -- all those questions. But there is no question that this happened because of someone who feels completely excluded, someone who feels completely at war with innocence, at war with society."

Harper harshly criticized Trudeau for trying to "to rationalize or make excuses" for the bombers.

In the U.S. on Saturday, however, authorities were leaving no stone unturned in trying to determine what might have motivated the suspects.

They're particularly interested in a trip the deceased Tamerlan Tsarnaev took to the semi-autonomous Russian province of Dagestan in 2012. The region has become a hotbed of Islamic insurgency.

They're investigating whether the 26-year-old boxing enthusiast was in contact with extremists or received any training from them while overseas.

Tamerlan Tsarnaev was not a stranger to the FBI. In a statement released late Friday, the agency said it had investigated the older Tsarnaev brother at the behest of a foreign government two years ago, and found he had no links to terrorist organizations.

"The FBI did not find any terrorism activity, domestic or foreign, and those results were provided to the foreign government in the summer of 2011," the statement said.

One official said Saturday that investigators believe the suspects were acting alone. Meantime, troubling portraits continued to emerge of the social isolation and increasing radicalism of Tamerlan Tsarnaev, in particular, amid repeated suggestions from friends and family that he may have led his younger brother astray.

Edward Deveau, chief of the Watertown police force, told CNN that early indications suggest the brothers were lone wolves, unassociated with any terrorist organization.

A special interrogation team was waiting to question the surviving Tsarnaev brother. But the U.S. citizen will be interrogated without authorities advising him of his right to remain silent and to obtain a lawyer, a warning typically given to crime suspects in the United States.

Called Miranda rights, authorities can revoke them if they deem the suspect poses a continuing threat to public safety. In Tsarnaev's case, that likely means investigators want to ensure there are no unexploded devices or accomplices still at large.

Dzhokar Tsarnaev was under armed protection at the same hospital where several of those wounded during his alleged bombings on Monday were still being treated. He's in serious but stable condition with gunshot wounds to the neck and leg, officials say, and not yet able to be questioned.

His brother died after a wild shootout with Watertown police early Friday that concluded with his younger sibling running over him in a carjacked SUV before abandoning the vehicle and fleeing on foot.

The brothers lobbed explosives at police during the encounter, including a pressure-cooker bomb similar to at least one device used in the marathon bombings. Its lid was found embedded in a car parked along a Watertown street, Deveau said.

Investigators tried to follow a trail of blood to find the suspect, but their search was fruitless until they lifted a "stay indoors" order for the area. Within an hour, a Watertown resident discovered Tsarnaev hiding out in a pleasure boat in his backyard.

One outspoken uncle of the Tsarnaev brothers -- and friends and associates of the suspects -- has publicly speculated that Tamerlan Tsarnaev had a sinister influence over his teenaged sibling, who has almost universally been described as a kind-hearted and intelligent young man with lots of friends.

Ruslan Tsarni, a brother of the boys' father, Anzor, told NBC News on Saturday that he suspected the boys were "just puppets and executors of something of bigger scale." A day earlier, he'd referred to them as "losers" who resented the success of others.

Tsarni added that he believed Dzhokar was "used by his older brother. He's just another victim of his older brother. He victimized others, but he's been used by his older brother."

Tamerlan, meantime, was likely radicalized by others while living in the United States, the Maryland resident said, pointing the finger at an unnamed Armenian associate. He also recalled a dramatic transformation in his nephew a few years ago.

"I was shocked when I heard his words, his phrases, when every other word he starts sticking in words of God," Tsarni said in the interview.

"I question what he's doing for work, (and) he claimed he would just put everything in the will of God. It was a big concern to me. He called me confused when I started explaining to him: 'Make yourself useful to yourself and to your family ....' It wasn't devotion, it was something, as it's called, being radicalized."

The suspects' parents, meantime, insist their boys were framed for the bombings, which killed three people, including an eight-year-old boy, and wounded more than 180, some grievously.

The bombings kicked off a frightening week for Americans still traumatized by the horrors of the Sept. 11, 2001 terrorist attacks. A massive fertilizer company explosion in rural Texas, just up the road from Waco and occurring almost exactly 20 years since the disastrous federal raid on the Branch Davidian complex, further compounded fears that the nation was once again under siege.

So far, that blast -- which killed 14 people -- appears to be an industrial accident.

The city of Boston erupted in relief and celebration when police captured Dzhokar Tsarnaev on Friday night after a homeowner emerged from his Watertown house and noticed something amiss with the tarp covering his boat. When he looked inside, he saw a body in a pool of blood and quickly called 9-11.

Heavily armed police surrounded the boat and tried to convince Tsarnaev to come out, but he wasn't communicative, said Boston Police Commissioner Ed Davis.

There was a final volley of gunfire between him and a sea of police officers, and Tsarnaev was taken into custody after he lifted his shirt to prove to police he wasn't rigged with explosives, officials said Saturday.

Shortly after Tsarnaev was whisked to hospital by ambulance, crowds gathered in downtown Boston to chant "U.S.A." The celebrations erupted just a couple of hours after authorities lifted their lockdown of the city and many of its suburbs following the intense, day-long dragnet.

The lockdown had kept almost a million citizens cooped up indoors, bringing the normally bustling city to an eerie standstill.

In addition to the three people killed in the bombings at the famed marathon, a security officer at the Massachusetts Institute of Technology was gunned down on Thursday night, about five hours after the brothers' photos were made public by the FBI.

A Boston transit police officer was also seriously wounded during the subsequent shootout in Watertown. Fifty-seven people remain hospitalized from the marathon bombings; several of them lost limbs in the blasts.