Painted on the bitumen below is the yellow finish line of the Boston Marathon, where two bombs killed three people and injured 238 nearly two years ago.

That morning Taylor blessed the athletes before they set off and returned to the church. Later she climbed the iron ladders that zigzag up inside its wonderful gothic revival bell tower to watch the finish from high above, as she does each year.

"I saw and felt and heard it all in the same moment," she recalls. "A kaboom and a lot of smoke."

She clambered down as fast as she could but was unable to get into the street to help because temporary fencing had effectively sealed off the church.

Sitting in her office this week as the trial of the surviving accused bomber, Dzhokhar Tsarnaev, grinds through its early days across town, Taylor confesses she is relieved now about that fence. She worries that confronted with the carnage in the street she would have been no help.

It is not the thought of the dead that bothers her, but all those smashed and severed limbs.

Bombs are terrible and indiscriminate weapons, and each type has its own character. The ones that hit the marathon were made with ball bearings and explosives packed into pressure cookers and they spat their shrapnel out low through the crowds on Boylston Street, scything through people's legs.

The start of the trial has caused disquiet among some observers, and renewed trauma to some of the survivors, Taylor says.

Like most other Bostonians, she does not believe the trial of Tsarnaev is really about his guilt or innocence. The evidence against him seems overwhelming. Instead most see it as a battle over his life. If the jury finds him guilty it must then decide if he should be put to death.

Boston is the liberal capital of a liberal state. Massachusetts has not put anyone to death since 1947, though capital punishment was not formally abolished in the state until 1982.

A Boston Globe poll conducted last September found that just a third of Bostonians support the death penalty in this case, with 57 per cent opposed.

But it is the federal government that is trying Tsarnaev, on 30 charges, 17 of which carry the death penalty.

Federal Attorney-General Eric Holder has directed prosecutors to pursue the death penalty, and they are doing so doggedly.

This has raised eyebrows in Massachusetts legal circles. "They wanted to try him and they found a way to do it," says Daniel Medwed, a law professor at Northeastern University, who fears the federal government might have over-reached its authority in the case. He believes there is a good argument that Tsarnaev should have been tried by Massachusetts.

Complicating the issue even more is that President Barack Obama and Holder are both known to be opposed to the death penalty.

So why is the federal government so keen to take Tsarnaev's life?

It is hard to exaggerate how profoundly the bombings affected this city. Boston, Medwed says with heavy understatement, is an interesting city.

It is where the Pilgrims landed in 1630 and where the American Revolution began almost a century and a half later. It has a proud working-class tradition, steeped in Irish Catholicism, yet it is famous as the home of two of the world's most elite places of learning, MIT and Harvard.

If anything weaves these strands together it is sport, and the Boston Marathon, held each year on Patriots Day, stands out as a unifying event. It is the most famous such race in the nation.

Nor does the death toll adequately reflect the horror of the bombings. Many more might have died had the blasts not taken place so close to top hospitals, had there not been first aid staff waiting at the finish line.

It is also easy to forget how dramatic and violent the pursuit and arrest of the two alleged bombers was.

On April 15, 2013, the two bombs exploded 13 seconds and a block apart on Boylston Street about 2.49pm, two hours after the marathon's winner crossed the line but before another 5700 runners had made it to the end.

As the FBI took over the scene Boston's airspace and airport were closed.

For three days the investigation progressed as parts of the mainstream and social media devolved into a frenzy of speculation and at times outright misreporting.

Finally, on April 18, authorities released images of the as yet unidentified Tsarnaev brothers, Tamerlan and Dzhokhar, in part to protect people who had already been wrongly identified by media.

Hours later, in nearby Cambridge, it is alleged that the brothers ambushed and murdered  27-year-old MIT Police Department officer Sean Collier, before carjacking an SUV and kidnapping its driver.

The end of the horror was long and began with a shootout in Watertown after midnight, when police and the brothers exchanged up to 300 rounds. Tamerlan was killed, apparently shot by police and then run over by Dzhokhar as he sought to escape.

One police officer was critically wounded, perhaps by friendly fire, and 15 were injured in the firefight.

The following day the western half of the metropolitan area was locked down and residents were ordered to "shelter in place" as members of at least eight city, state and federal agencies, as well as the military, searched for the brothers. It was the first test of inter-agency task force protocols developed after September 11. The public transport system was closed, as were schools, universities and many businesses.

What might seem a natural precaution in some parts of the world unsettled many in the United States, who did not like to see a population acquiesce with such docility to a government order. A columnist for Israel's Ha'aretz newspaper later observed, "More than a million people cowered in their homes on Friday, under curfew, gripped by fear. The US army and other security forces invaded and occupied an American town. Helicopters, armored vehicles, soldiers in full battle gear and crack SWAT teams sealed off streets and conducted house-to-house searches."

The following night Dzhokhar Tsarnaev was found by a resident, cowering in a pool of his blood in a boat in a back yard. He had allegedly scrawled messages while hiding. "The US government is killing our innocent civilians", "I can't stand to see such evil go unpunished", and "We Muslims are one body, you hurt one you hurt us all". He has barely been seen since.

The United States is defending Tsarnaev, who is now 21, as fiercely as it is prosecuting him. His defence team is lead by Judy Clarke, known as one of the nation's best death penalty lawyers, if not the best.

She is famous not for winning acquittals for high-profile clients, but for saving their lives with plea deals. She won a deal for Unabomber Ted Kaczynski, and saved the life of Jared Lee Loughner, who shot US Senator Gabrielle Giffords in the head in a mass shooting that killed six and injured 13 in 2011.

Clarke is backed by a team of Massachusetts' best defence lawyers. As Medwed puts it, the Justice Department is determined that this shall be a good trial.

What has surprised Medwed, and others, is that Clarke has been unable to negotiate a plea deal for Tsarnaev despite Obama and Holder's reservations about the death penalty.

Whatever your views on capital punishment, there are strong arguments for such a deal, he says.

Should he have been allowed to plead guilty to spare his life, Tsarnaev would have already vanished into life imprisonment.

Prosecutors' pursuit of the death penalty means that not only will this trial dominate news in Boston for months, but he will automatically be granted a series of appeals over many years. He will be in the public eye for a generation unless a deal is cut.

But Holder's prosecutors have cut no deal. Many observers, Medwed included, believe the federal government's aggression in this case is in part bound up with what is going on in Guantanamo Bay, Cuba.

Campaigning for his first term, Obama promised his first act as president would be to close the United States' secret prison in Cuba, which he believed served to motivate terrorists around the world.

Obama wanted to bring the detainees back to the US to face the civil justice system. Republicans blocked the move in Congress. But he has not given up on the closure and his administration recently fast-tracked the release of detainees not believed to pose a continued threat.

It could be that Holder and Obama believe that the enormity of Tsarnaev's alleged crime warrants his death, but the administration has clear motivation for proving to observers that the civil court system can successfully try a terrorism case and extract the ultimate penalty.

The federal court building in Boston is an unlovely brick hulk of place not made any prettier on Thursday by a dusting of snow. In a courtroom on the third floor the prosecution and defence teams – Tsarnaev among them – faced each other over a wide table as potential jurors were led in and questioned over questionnaires they had completed earlier.

Tsarnaev can't have missed the banality of much of this part of the fight for his life.

A woman was asked if her daughter's boyfriend's service in Afghanistan might colour her view of the case. No. Did she use social media? She liked to text her sister.

But the common theme was the death penalty. The defence sought to weed out fervid supporters of capital punishment, while the prosecution picked off those ideologically opposed. If the system works right, Medwed says, the jury will be made up of those in the "mushy" centre – people who might be convinced either way.

They might be hard to find in Boston.

The first juror questioned said he believed Tsarnaev was guilty and would sentence him to death. Asked by Judge George O'Toole jnr if he might be swayed by evidence he said, "I can't imagine any evidence that would change how I feel about what happened".

O'Toole has already denied Clarke's attempts to have the trial's location moved and its start delayed because of the attacks in Paris. Medwed said this manoeuvering too was about the death penalty. Clarke was putting the matters on the table so they could be reconsidered in a future appeal.

Next week the lawyers are expected to present their opening statements. It is anticipated that Clarke will describe Tsarnaev as a young man in thrall to a domineering older brother.

Prosecutors will describe a case bolstered by evidence that includes not only a confession and eyewitness accounts but by surveillance footage of the brothers placing the two bombs on Boylston Street, one of them next to the youngest victim of blasts, eight-year-old Martin Richard.

His loss in particular shattered the community, and Taylor said the trial had reopened those wounds and the pain had been compounded by the attack on Charlie Hebdo, also perpetrated by brothers.

"It just makes our hearts break more, break again."