One was alienated and increasingly drawn to Islam. The other was a skateboarding dope smoker. A complex picture is emerging of the Boston bombers

The behaviour of Dzhokhar Tsarnaev, the surviving suspect in the Boston marathon bombing, was not what might have been expected from America's most wanted man, nor from a jihadi determined to perish fighting the United States.

For the first three days of last week, the 19-year-old attended the University of Massachusetts in Dartmouth, where he was studying. He worked out at the gym, and on the Wednesday night he slept in his room. According to friends, he attended a party with friends from his football team. "He was just relaxed," one witness said.

A day later, his elder brother, Tamerlan, 26, who on Friday would die from the wounds sustained in a gun battle with police, was also not keeping the lowest of profiles. The brothers' uncle Ruslan Tsarni, 42 – who called them "losers" who had failed to fit in – told the media that, on the night before he was killed, Tamerlan had called Ruslan's older brother, the boys' father. "He said to my brother the usual rubbish, talking about God again, that whatever wrong he had done on his behalf, he would like to be forgiven," adding, "I guess he knew what he had done."

Within 24 hours of that call, the two young men, ethnic Chechens born in Dagestan who had spent the last decade in the US, were being hunted through Boston, with the whole city ordered to take shelter behind closed doors after they had been identified by video from security cameras as they apparently planted their bombs.

Their behaviour in the period between the detonation of the bombs, which killed three people and injured almost 200 more, and the denouement of the hunt that led to Dzhokhar's capture – hiding wounded in a boat in a backyard – seems as baffling as the motives for their attack in the first place.

For while Tamerlan – in some respects at least – appears to have fitted the profile of an alienated young man who had drifted towards an ever-more fundamentalist version of Islam, his skateboarding, dope-smoking younger brother seemed the antithesis of that image. It is this that has led many analysts already to suggest that the actions of the Tsarnaev brothers has more in common with alienated and unhappy "school shooters" – caught up in their own narcissistic drama – than typical Islamist extremists.

One acquaintance of Dzhokhar – known to friends as "Jahar" – who emailed the blog of well-known US commentator Andrew Sullivan, speculated he had fallen under his brother's influence. "I graduated a year before … Jahar, and while I wasn't very close with him, I knew him fairly well. While it seems like every time this happens people say I can't believe this happened, this truly is a case where the personality and the behaviour don't seem to match up. Jahar was quiet, but fairly social. He came to parties, smoked weed with me and many of my friends and was always cheerful.

"My feeling is that the reason that Jahar was involved has entirely to do with his brother ... Given that his brother essentially raised him, I think this is an awful case of evil being perpetuated because of the trust and love Jahar had for his brother."

The Twitter account of Dzhokhar over the last few months provides, if not clues to his mindset, at least some suggestions. He was growing tired of America and missed Dagestan. In January he wrote: "I don't argue with fools who say Islam is terrorism it's not worth a thing, let an idiot remain an idiot."

Those who trained with Tamerlan, a talented young boxer known to them as "Tom", seemed as surprised as his brother's friends were that he had emerged as prime suspect in the bombing.

"In the ring, he could knock a man out with one punch,'' Gene McCarthy, founder of the Somerville Boxing Club, told the Boston Globe. "But when he sat at a piano, he could play classical music like you wouldn't believe. The Tom I knew was a sweetheart.''

That Tamerlan changed in recent years is clear. "He was not devout, practising. But about three years ago he began praying five times a day," his aunt Maret Tsarnaeva told CNN.

And although Tamerlan had once embraced life in the US, even hoping to box for his adopted country at one stage, more recently he had seemed unhappy in America.

"I don't have a single American friend. I don't understand them," he was quoted as saying in a photo package that appeared in a Boston University student magazine in 2010.

He identified himself then as a Muslim and said he did not drink or smoke: "God said no alcohol."

Albrecht Ammon, 18, who lived directly below the flat shared by the brothers, said he recently saw Tamerlan in a pizzeria, where they argued about religion and US foreign policy. Tsarnaev argued that many US wars are based on the Bible, used as "an excuse for invading other countries". But even then he added he had nothing against the American people.

Video believed posted on YouTube by Tamerlan – including links to radical Islamist material – told a darker story still, as did the fact that the FBI had interviewed Tamerlan at the request of a "foreign government" – believed to be Russia – over suspected Islamist extremist views but found nothing to be alarmed by.

Whatever finally emerges as the underlying cause which persuaded the two young men to launch their murderous attacks – a sense of alienation, jihadi motivation or obsession forged in the midst of the their fraternal relationship or a combination of all three – by Friday night, Tamerlan would be dead and Dzhokhar surrounded in the town of Watertown.

They had been tracked down as much because of the efforts of a 58-year-old electrical engineer named Bob Leonard who, when the first fuzzy images of the brothers appeared, realised he had clear pictures of their faces at the marathon which he gave to the FBI – images that would be all over US television the following morning. They were the pictures that would shatter the sense of surreal calm and outward normality of the brothers' existence in the period immediately after the marathon bombings.

The photographs, and an accompanying video of the bombing suspects walking through the marathon crowd carrying dark rucksacks, quickly went viral.

Their behaviour indicates that the brothers knew that the thousands of FBI agents and police officers hunting them were closing in. But as they set off on a journey that would end with the elder brother dead, the younger brother bloodied but alive – after an extraordinary 24 hours of mayhem – their acts betrayed the same puzzling combination of brutal planning and casual insouciance that they had showed earlier in the week.

The brutality came first. At about 10.30pm on Thursday, just five hours after their images were released by the FBI, the Tsarnaev brothers are believed to have jumped a police officer on the campus of the Massachusetts Institute of Technology.

Sean Collier, 26, had answered reports of a disturbance at MIT and had parked his police cruiser outside the Stata Center. Details are sketchy about what happened next, but it appears the suspects attacked Collier unprovoked in what was described as an ambush. He died on arrival at hospital.

They carjacked a Mercedes SUV and forced the owner to drive them west along Memorial Drive, heading out of town. But instead of killing him with the same lack of compunction they had shown Collier, they let him go at a petrol station, even giving him an instruction: "Tell the police that we did the bombing." It was as though they wanted to bring it on.

As they drove along Memorial Drive, at some point they stopped alongside another car and spent some time transferring objects out of it into the stolen Mercedes. The objects included several pipe bombs as well as guns.

The owner of the Mercedes duly followed orders and called the police, sparking a massive car chase that ended in that hail of gunfire in Laurel Street in the heart of the quiet suburb of Watertown. When they came to make their final stand, the siblings displayed a notable, and perhaps important, contrast in behaviour.

Tamerlan went down fighting, hit by police bullets. According to the police he was wearing a suicide vest. Martyrdom appeared to be his chosen option.

Not so the younger Dzhokhar, who made his escape hijacking another car and driving away from the fire fight before fleeing on foot. As if to underline the difference, as he sped away Dzhokhar drove over his stricken elder brother.

On Friday the residents of Watertown had been on lockdown for most of the day – like much of Boston – as Massachusetts governor Deval Patrick urged people to stay in their homes. Ringed-off by police tape, its boundary was patrolled by state police troopers. Inside the deserted police zone, armoured Humvees could be seen in the streets, while every shop, cafe and restaurant was closed. There were no civilians walking the streets, no cars on the road. The only signs of life were sporadic police checkpoints, state troopers stood beside their cars. In a day devoid of information and full of confusion, they knew as little as anyone else.

With no sign of Dzhokhar, Watertown residents were finally told they could come outside just after 6pm. Among them was Sara Zirolla who decided, with her mother Emilia, to go for a walk round the neighbourhood. They had got one block, walking past the end of Franklin Street when they heard gunfire close by.

"[There were] Many, many, many shots," said Emilia Zirolla, 52. "More than 15."

The Zirollas ended up standing with 50 other locals behind police tape, craning their necks for a glimpse of what might be happening. Swat teams arrived, as did police and Watertown firefighters. Several ambulances arrived on the scene.

As this unscheduled neighbourhood gathering continued, at least nine loud bangs were heard at around 8pm, stun grenades being thrown at the boat where Dzhokar was taking shelter, injured in the stern under a tarpaulin.

What is now clear – as the FBI and police entered this final stage of an extraordinary week-long manhunt – is that their priorities noticeably shifted. If up to that point their focus had been on public safety, going to unprecedented lengths to avoid further loss of life by putting the whole of Boston into virtual lockdown, now they knew that Dzhokhar was their best hope in helping them tackle some of the unanswered questions of this most baffling of weeks. What the Zirollas heard was officers, under fire from Dzhokar hiding in a boat, responding with non-lethal bullets and flash grenades designed to daze him into submission. All the while a helicopter buzzed overhead equipped with infrared technology to detect what the teenager was doing underneath the tarpaulin; hostage negotiators were used to persuade him that he was in need of urgent medical attention. Finally, an FBI hostage rescue team was sent in, entering the boat and taking Dzhokhar alive.

Governor Patrick explained the imperative to take him alive at a press conference on Thursday.

"I hope very deeply he survives those wounds," Patrick said as Dzhokhar was being rushed to hospital. "Because I've got a lot of questions and I know investigators have a lot of questions for him."

"I used to warn Dzhokhar that Tamerlan was up to no good," Zaur Tsarnaev, a 26-year-old cousin from Makhachkala, Russia, where the brothers briefly lived, told the Boston Globe. "[Tamerlan] was always getting in trouble. He was never happy, never cheering, never smiling. He used to strike his girlfriend … He was not a nice man."