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 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.