It’s a shocking image: Two young girls, skinny teenagers in brightly colored salwar kameezes, hang from opposite branches of a tall mango tree, suspended from their necks by their own pink and green dupattas. Their eyes are closed. Their heads, bent by gravity, appear bowed in reverence toward one another. Sari-clad women sit in a circle around the tree, several clasping their faces with their hands. A few hold small children.

It’s a snapshot of the scene in the village of Katra in Budaun, Uttar Pradesh, on May 28, 2014, the morning after the girls — Indian law requires that their names be withheld, so journalist Sonia Faleiro calls them Padma and Lalli, cousins “alike as two grains of rice” — went missing in the fields behind their homes after heading out for a “final squat” for the night.

Just two days before, newly elected Prime Minister Narendra Modi was sworn in with promises of prosperity. “Achhe din aane waale hain” — good days are coming soon — he said. The national mood, Faleiro notes, was predominantly one of triumphant ascendancy. “India had been poor for so long; but now, rather than being dismissed as a lumbering elephant, it was a tiger,” she writes, and “as people’s vanities were stoked, their appetite for news stories about girls like Padma and Lalli shrank. . . . The world was watching India, but no one was more bewitched by the transformation than Indians themselves.”

It may have not fit the prevailing narrative, but back in Katra, it was hard to ignore the spectacle of the girls’ deaths. In the hours after they were discovered in the mango grove, Padma’s and Lalli’s distraught families refused to let the police bring the bodies down. As videos and photos of the dead cousins went viral (“They looked like ‘dangling puppets,’ ” one local recalled), tourists lined up to see them, blocking the road to the village with their horse carts, motorbikes and tractors. Village children dug a makeshift helipad in the dirt for the arrival of high-profile politicians. Family members gave news conferences, demanding justice.

Who or what killed the girls? Initial accounts from relatives indicated that 16-year-old Padma and 14-year-old Lalli had been abducted by a group of men from a neighboring village; a hasty post-mortem suggested that the cousins had been raped before they died. In the aftermath of the brutal 2012 Delhi bus rape case, the story, at first, seemed horrifying but familiar: “Two girls found gang-raped and hanged in India,” a matter-of-fact Washington Post headline from May 29 declared. The truth, as London-based writer Faleiro gradually discovers in “The Good Girls: An Ordinary Killing,” was considerably more complicated.

For a previous book, Faleiro immersed herself in the underworld of Mumbai’s secret dance bars. Here, she takes a more forensic approach, starting in Katra and patiently working her way across the region over the course of four years. In her search for answers, Faleiro interviews more than 100 people and combs through some 3,000 pages of records from India’s Central Bureau of Investigation. It’s a remarkable feat of reporting: What she finds reveals as much about the failings of India’s law enforcement, media and politics as it does about the girls’ deaths.

Right away, it’s apparent that caste and clan politics are central to this story. Lalli’s and Padma’s families are Shakyas; the chief suspects, a 19-year-old named Pappu whom the girls knew from the neighboring village, and his brothers, are Yadavs. The Shakyas and the Yadavs both fall into the category of “Other Backward Classes,” or OBCs — historically oppressed low-caste groups. But there is a key distinction: In Uttar Pradesh, Yadavs wield political power. In 2014, the state’s chief minister was a Yadav with a reputation for corruption, backed by numerous Yadav law enforcement officers. The Shakyas’ mistrust of the Yadavs, and more generally of the powerful, ran deep, Faleiro explains: “It was easy to point to earlier acts of grave malfeasance — by police officers, even the prime minister — to show that in India anything was possible, and nothing was what it seemed.”

As Faleiro probes the case, an extensive supporting cast emerges: meddlesome uncles, drunken police officers, hopelessly unqualified coroners, sensationalizing TV newsmen, a sneering intelligence officer and grandstanding politicians, all with a part — however undignified — to play in this story. (Faleiro’s prose is restrained, but she allows the occasional colorful simile, as when one unreliable witness is described as “coming apart like overripe fruit.”) Everyone agrees that the girls’ deaths are a tragedy; no one knows quite whom to blame. Or, as Faleiro puts it: “Everyone agreed the system was rotten but no one knew how to fix it.” That same year, one child went missing in India every eight minutes.

“The Good Girls” isn’t the first book of nonfiction to make a close study of the fates of India’s most destitute citizens; in recent years, there has been a steady stream of them. Aman Sethi’s “A Free Man: A True Story of Life and Death in Delhi,” Katherine Boo’s “Behind the Beautiful Forevers: Life, Death, and Hope in a Mumbai Undercity” and Faleiro’s own “Beautiful Thing: Inside the Secret World of Bombay’s Dance Bars” — among others — each applied a similar rigor to their explorations of the lived experience of poverty on the subcontinent. But as Faleiro explains in an author’s note, “The Good Girls” was supposed to be something else. The book she set out to write was intended to be an investigation of the nature of sexual violence in India.

Instead, what she finds is a story of the constraints placed on “ordinary” girls’ lives from an early age. It’s the story of the girls’ larger-than-life “honor” — a disembodied thing that hangs about them menacingly, threatening to fall away at any moment, ruining the whole family. The girls were often seen clutching a cellphone — an obvious threat to their honor. In some villages in Uttar Pradesh, unmarried women were forbidden from using mobile phones. (Undoubtedly the girls were “romancing someone,” one neighbor concluded.) The day they disappeared, the girls attended a local fair, where they ate greasy pakoras and bought nail polish — again, risking their honor (after all, “it wouldn’t do for girls to be seen enjoying themselves in a public place”).

That word — “enjoy” — takes on a dark shade in this book. It’s a word that can be heard on a short recording of one of the last phone conversations Lalli had, with Pappu, on the day the girls died. But what enjoyment meant, really, to Padma and Lalli is something Faleiro can’t entirely uncover. After their bodies are found, the men of their families hide and then destroy the very items that could crack open the case: the mobile phones the girls used, which, thanks to software requested by Lalli’s father, held recordings of their calls. “Who knows what’s in the phone,” Lalli’s father says — but whatever it was surely “should be deleted.”

Faleiro lets the suspense build as she carefully uncovers the villagers’ competing motives. Gradually, it becomes clear that in Katra, ultimately one thing is more binding than police codes, medical codes or penal codes: a retrograde but resilient code of honor. This is the force, above all others, that stunted the girls’ lives and hastened their deaths. In the end, Faleiro writes, “an Indian woman’s first challenge was surviving her own home.”

The Good Girls

An Ordinary Killing

By Sonia Faleiro

314 pp. $26