SHAH PORIR DWIP, Bangladesh >> From his shop overlooking a pier on this island near the border with Myanmar, Mohammad Hossain watched the human smuggling business swell.

Over the years, the trickle gradually grew into an unending stream. The late-night flashes of light on the water, signaling that the coast was clear to launch boats, multiplied until they looked like summer lightning. That the boats were not carrying fish was an open secret here: One day, when a trawler sank on its way out, the water was littered with floating human bodies.

The people of Shah Porir Dwip — fishermen, shopkeepers, police officers and shadowy bosses — were all drawn in, as participants or concerned observers, to a multimillion-dollar people smuggling business that sent roots deep into this impoverished corner of Bangladesh.

The outside world came to know of the smuggling this spring, through a series of awful revelations. Shallow graves were discovered in makeshift camps in Thailand, near the Malaysian border, where smugglers abused and starved their captives, demanding as much as $3,000 from their families for their release. Boats were abandoned in the middle of the ocean, packed with people on the edge of starvation.

Here, there was neither horror nor surprise.

Smuggling initially focused mainly on fortune-seeking migrants and offered local people a lucrative alternative to traditional livelihoods like fishing and salt mining. But it has expanded into a predatory operation of considerable scale. Traffickers have used high-pressure tactics to pack rickety boats with desperate refugees, especially ethnic Rohingya fleeing poverty and persecution.

They sometimes worked with migrants who had made the journey to Malaysia and then targeted their own relatives and friends, in the style of a pyramid scheme, pocketing large fees as new bodies were fed into the network. Stories of abductions became common. The operation grew, blanketed in silence.

Watching it became a burden to the shopkeeper, Hossain, who spoke to the migrants for a few minutes as they bought packets of cookies on their way to houses where they would hide, waiting for the signal to go to the boats. His sundries shop was their last stopping point before the journey, and it weighed on him.

“I told some of those young men, ‘Don’t go,’” he said in a recent interview. “I told them many boats sank on the way. But they didn’t listen. They had a dream. They wanted to earn money abroad.”

The story of the smuggling route is also the story of trapped people.

Most of the people fleeing were Rohingya, members of a Muslim ethnic group who years earlier fled Myanmar, where the government considers them intruders from Bangladesh. But their lives were little better in Bangladesh, one of the world’s poorest countries, where many of their neighbors despise them and also consider them outsiders.

Shah Porir Dwip, a coastal town of around 60,000 people, is at the mouth of the Naf River, which separates Bangladesh from the misty mountains of Myanmar. It was across this mile-wide river that hundreds of thousands of Rohingya poured into Bangladesh in 1992, fleeing a campaign of arson, rape and extrajudicial killing by the Burmese army.

Those Rohingya migrants who refused to return and their descendants — 32,000 registered refugees and an estimated 300,000 unregistered — have lived for 23 years in a kind of limbo in Bangladesh, occupying squalid camps and constituting a vast illegal workforce.

It was one of these stateless people, a Myanmar-born Rohingya fisherman named Tazer Muluk, who in 2000 discovered a way out, a sea route to Malaysia that could be made in less than a week, the Bangladeshi police concluded in an investigation last year. By 2014, the network had become sprawling, including at least 600 smugglers and 1,600 low-level agents and boatmen.

Mohammad Ataul, a 26-year-old Rohingya who had grown up in one of the camps, occupied the bottom spot among 15 tiers of agents, the one whose job it was to persuade people to make the journey. His pitch began with simple math: A day’s work in Malaysia would bring three times as much as the 300 taka (about $4) that undocumented laborers could earn in Bangladesh. Very little persuasion was necessary, he said, and safety was never the primary concern.

“There is no guarantee on the sea,” he said. “I would tell them, if you don’t die in the sea, you will end up having a job.”

In a year and three months, he and his partner recruited about 400 people, splitting a fee ranging from around $40 to $65 per head, so, as he put it, “if I can get more people I earn quite good money.” Three days before each journey, the agents routinely delivered a per-head payment to the police or border guards.

“Everybody gets money out of it,” he said. “Everybody knows.”

It is difficult to say exactly when the business turned predatory, but Abdul Hamid, who owns a pharmacy in the town of Ukhia, about 40 miles north of here, recalls a sultry night last summer when, just as he was closing his storefront, a terrified man dashed in and threw his arms around Hamid’s legs.

Hamid had noticed some fluctuations in his business: He was selling unusual amounts of sleeping pills and surgical tape. Now, he gazed down at the man at his feet, his back and shoulders bruised, who said he had escaped smugglers as they tried to force him into a boat. When Hamid began to ask around, he was surprised at how many local men had disappeared; kidnapped, Hamid surmised.

“When we came to know the picture, it was extremely scary,” said Hamid, who began to organize rallies to condemn human smuggling. “It was not trafficking. It was like selling and buying people. Like animals.”

Activists monitoring trafficking in the Andaman Sea, west of Thailand and Myanmar, also noticed a leap in volume that began at the end of 2013. If one or two large boats had been making the journey earlier, now there were as many as 25 large ships at one time, said Chris Lewa, the director of the Arakan Project.

The Office of the U.N. High Commissioner for Refugees calculates that 25,000 migrants left the Bay of Bengal in the first three months of this year, double the number who left in the corresponding periods of 2014 or 2013.

“The boats got bigger, the sailing season got longer, the stakes and the money got higher,” said Joe Lowry, a spokesman for the International Organization for Migration.

Now, the traffickers were looking for new ways to fill their boats.

Ataul’s duties grew to include extortion, a payment option for those unable to afford the $2,500 for the journey. These migrants, he said, would pay a small amount upfront, then be held in a camp in Thailand while the smugglers sought ransoms of about $2,500 to $4,000 from their families. About a third of the families were able to come up with the money, he said; the rest of the migrants were starved and abused, sometimes for months.

“I feel bad about it,” he said. “I started not knowing that this was going to happen.”

It was his job to collect the ransom money from relatives, who often wept and begged him for help. He began changing his telephone number every month or so, to avoid the calls. Meanwhile, more and more of the people he recruited — probably 40 percent overall — ended up in police custody before they could reach their destination.

Increasingly, the migrants boarding boats for Malaysia were not stateless Rohingyas, but ordinary Bangladeshis, who had heard rags-to-riches stories from Malaysia. By early this year, Bangladeshis made up 40 to 60 percent of the migrant traffic, according to the U.N.’s refugees agency.

Choipu Sing, 55, a Bangladeshi citizen, watched enviously when a Rohingya friend returned from Malaysia, seemingly flush with cash. Sing’s friend made a tantalizing offer: He had a lucrative fish business and was seeking a partner to split labor and profits.

When Sing showed up for the journey, though, he realized he was not a partner.

The agents forced him to strip and pushed him below deck into a metal enclosure that held 182 people, he said. The ship headed into a fierce storm, leaving the migrants to toss in the baking hold, vomiting and weeping. At some point, Sing said, he fought his way up to the deck to gulp a breath of fresh air. But when he emerged into the light, crew members beat him with bamboo sticks so that he was forced back below, onto the mass of stinking bodies.

In all the suffering and humiliation that followed, what stung him the most was the betrayal of his trust. Asked what he would say to his old friend if he saw him again, Sing looked stony.

“If I see him again, I will eat him raw,” he said.

During the first five months of this year, a period that ended just as the mass graves in Thailand were discovered, the Bangladeshi authorities arrested 340 people on suspicion of smuggling, according to Mirza Abdullahel Baqui, who heads the trafficking unit of Bangladesh’s criminal investigations department. He said most were agents or owners of fishing boats, men who had become very wealthy recruiting migrants for the journey.

There have been no arrests of politicians, police officers or border security forces. Because the network was active for so long, “it is likely that some individuals in the administration and Police Department were involved,” said Baqui, adding that investigators were looking into police involvement.

In Shah Porir Dwip, the police were arriving in small boats nearly every day to take away townspeople identified as smuggling agents. But the men they arrested were poor, fishermen’s helpers and laborers, and even the local police chief chuckled at the low number of arrests.

There was one arrest no one laughed at.

In mid-May, the police detained Dholu Hossain, a jowly Bangladeshi fisherman described by neighbors and the authorities as one of the smuggling godfathers of Shah Porir Dwip. The police took him away one morning, said his wife, Amina Begum. Hours later, they said that Hossain had drawn a weapon and had died in a shootout.

It was one of five shootings that would take place under similarly mysterious circumstances. Baqui said the deaths occurred while officers were trying to carry out arrests.

The shopkeeper, Mohammad Hossain, described Dholu Hossain’s death as a targeted assassination, and he said he approved.

“We are very happy,” he said. “If the government kills 400 agents, 160 million people in Bangladesh are going to be happy.”

And those dreaming of escape hope that when the monsoon seas quiet in a few months, the police pressure will ease and the networks will revive.

Tahera Begum, an 18-year-old Rohingya woman, was in a warehouse, awaiting the signal to board a ship to Malaysia, when the scandal erupted in May.

Now she waits in the camp, hoping for another chance. She has heard the stories from migrants who survived the journey, she said in an interview, but if the boats start again, she will go in a second.

“I will try again,” she said. “I am afraid, but I will still try. I want to get out.”

© 2015 The New York Times Company