New Delhi (CNN) -- As many as 130 people may have drowned off the coast of southern Bangladesh after a boat carrying passengers trying to illegally get into Malaysia sank in the Bay of Bengal, Bangladeshi authorities said Thursday.
The accident appears to be the tragic result of an annual migration of thousands of people along dangerous, clandestine routes operated by criminal gangs.
Details have only recently begun to emerge about the sinking, which happened early Sunday, according to Lt.-Col. Zahid Hassan, the commander of the Bangladeshi border guard battalion in Teknaf, the area from which the boat is believed to have departed.
Just six survivors have been recovered by Bangladeshi authorities from the 136 people reported to have been on the vessel, Hassan said. Authorities are seeking nine people identified as operators of the human trafficking ring that organized the boat trip, he added.
Most of the people on board are believed to have been Bangladeshi, according to Hassan. But some, including three of the survivors, were Rohingya, Muslim people from Rakhine state in Myanmar, or Burma.
After lethal clashes this year between Rohingya and Buddhist residents of Rakhine, thousands more have tried to escape across the border only to find their path blocked by Bangladeshi authorities who say there are already too many refugees in the country.
Chris Lewa, director of the Arakan Project, a Bangkok-based NGO focused on Rohingya issues, said that local contacts told her another seven survivors from the boat sinking had been found on the Myanmar side of the border.
The survivors were taken to the town of Maung Gaw by fishermen, she told CNN. The fishermen also reportedly found six bodies, including two women, who they believe to be victims of the boat accident.
Lewa said local sources told her that boats chartered by relatives of the victims had also seen 40 bodies floating at sea on the Burmese side of the border, roughly 20 miles south of Bangladesh.
However, fishermen are too frightened to retrieve the bodies because they fear questioning by Burmese authorities, she said.
"That is the unfortunate state of the situation there now," she said. "There is a chance that there may be many survivors on fishing boats but most are likely to go back into hiding as they are too frightened to come forward."
More than 230,000 Rohingya refugees are estimated to be living in camps in Bangladesh, but only about 30,000 of them are officially registered. The refugees began arriving in large numbers in the early 1990s, fleeing what they said was persecution by Myanmar's military rulers at the time.
Teknaf, from where the boat is said to have set off, is on the banks of the Naf River, which forms the border between Bangladesh and Rakhine.
The people who are willing to pay traffickers operating the illegal sea routes to Malaysia tend to be impoverished Bangladeshis seeking to earn better money abroad and Rohingyas who see no way to build a life in Bangladesh, said Dirk Hebecker, the head of the U.N. Refugee Agency's office in Cox's Bazaar, Bangladesh.
"We consider this journey extremely risky," Hebecker said. "It is usually young males between 18 and 35 who take to the boats and leave their families behind in Bangladesh."
The U.N. agency estimates that between 50 and 150 people leave Bangladesh by boat each week, and the number may even have risen as high as 200 per week recently. Due to the clandestine nature of the departures, there is no systematic data collection.
Bangladeshi authorities believe that Rohingyas account for between 10% and 20% of the passengers on such journeys, Hassan said.
Hassan said the boat that sank Sunday went down after water filled its engine casing though a crack apparently caused by a collision with an undersea rock, citing the account given by one of the survivors to Bangladeshi authorities. It was on its way to meet up with a larger ship that would illegally transport the passengers to Malaysia, he said.
The journey usually takes between four and five days, depending on the boats and the navigation skills of the boat crews, according to Hebecker.
The occupants of those boats that do make it to Malaysia -- or Thailand, another possible destination -- often either end up in exploitative labor arrangements or detention, Hebecker said.
"They have no official documents as they are smuggled into these countries, and often have to cover substandard accommodations and food at low wages," he said.
CNN's Harmeet Shah Singh reported from New Delhi; Elizabeth Joseph and Jethro Mullen reported from Hong Kong; Brian Walker contributed from Atlanta.