Two boats carrying an estimated 530 Rohingya refugees disappeared after leaving Myanmar's Rakhine State on June 29, with aid agencies believing both vessels likely capsized in rough monsoon seas.
With no confirmed survivors, limited communications in conflict-hit Rakhine and no official search-and-rescue operation.
Investigators have been unable to determine exactly where or how the boats were lost.
More than 500 Rohingya refugees are feared dead after two boats carrying an estimated 530 asylum seekers disappeared in the Bay of Bengal in what the United Nations says could be one of the most fatal maritime misfortunes involving the persecuted minority in recent years.
The boats left Myanmar's conflict-ridden Rakhine State on June 29 and have not been heard from since. Humanitarian agencies believe both boats capsized during the brutal monsoon season, though the exact circumstances remain are yet to be known.
According to the UN refugee agency, International Organization for Migration, and humanitarian groups, the two boats departed from Sin Tet Maw, a village in Myanmar's Rakhine State.
″While the incidents and casualty figures have yet to be officially confirmed, UNHCR and IOM are gravely concerned by the potentially devastating loss of life,” the agencies said.
One vessel carried around 250 people and left in the morning, while the other boat with about 280 passengers departed later on the same day. Most of those onboard were Rohingya fleeing persecution in Myanmar, while some had travelled from refugee camps in neighbouring Bangladesh.
The boats are believed to have been headed to southern Myanmar before passengers were to be taken to Thailand to Malaysia through land. The latter has become home to a large Rohingya diaspora and the preferred destination for many refugees.
What is believed to have happened?
Both boats vanished shortly after departing.
The first is believed to have lost contact only hours after leaving Rakhine. The second is thought to have sunk off later on Myanmar's Ayeyarwady coast around July 8.
The UN has not officially confirmed the sinkings or the death toll. However, aid organisations believe both vessels likely capsized amid rough monsoon conditions.
The Bay of Bengal and Andaman Sea become especially dangerous during the monsoon season, with heavy rain, strong winds and high waves making the already hazardous crossings even more dangerous. Furthermore, the boats used by smugglers are often old fishing trawlers overloaded with passengers and equipped with unreliable engines.
Why do aid groups believe the boats sank?
Humanitarian workers have come up with the likely sequence of events from scattered reports.
While the first communication is usually first established within seven to ten days, there has been no update on contact in these two cases. A woman's body later washed ashore in Bangladesh, while fishermen recovered several more bodies between Myanmar's Irrawaddy Delta and Mon State days later.
Although no wreckage has been recovered, campaigners monitoring Rohingya migration say the location of the bodies and the prolonged no contact strongly suggest both boats capsized. If the vessels did sink, experts believe survival chances would have been extremely low.
The boats were travelling during one of the roughest periods of the year, carrying hundreds of passengers, many of them women and children, with little or no safety equipment. Unlike commercial vessels, migrant boats rarely carry emergency communication systems. The route also passes through remote stretches of sea where search-and-rescue operations are limited or non-existent.
No government has announced a coordinated search effort, and no confirmed survivors have emerged weeks after the boats disappeared.
Why is the disappearance so difficult to investigate?
The conflict in Myanmar has made independent verification nearly impossible. Another concern is the brutal monsoon that could have hampered their route. The Rohingya usually avoid making their journey in the monsoon.
Rakhine State has become akin to a war zone, with the Arakan Army controlling most of the territory while Myanmar's military retains only a handful of positions, including the state capital, Sittwe. Communications across much of the region have been severely disrupted, cutting off communication with local communities.
Humanitarian organisations rely largely on informal networks of contacts, relatives of passengers, fishermen and local activists to reconstruct events.
Smuggling operations are also highly secretive, leaving few official records of departures, passenger numbers or routes.
The boats were not believed to be heading directly to Malaysia.
Instead, experts say they were likely travelling south along Myanmar's coastline, where passengers would have disembarked before being moved through clandestine transit camps and overland routes into Thailand and eventually Malaysia.
Smuggling networks operating across Myanmar, Bangladesh, Thailand and Malaysia have increasingly adopted this route as direct sea journeys to Malaysia have become more difficult due to maritime interceptions.
Why are more Rohingya attempting these journeys?
Almost 1.2 million Rohingya refugees remain in camps in Bangladesh after fleeing Myanmar's military crackdown in 2017, which the United States has described as genocide. Another estimated 600,000 Rohingya continue to live in Myanmar under severe movement restrictions, with many confined to internment camps.
Conditions seem to have gotten worse in recent months. Food scarcity has become prevalent amid aid cuts by donor countries. Fighting between Myanmar's military and the Arakan Army has also intensified in Rakhine State.
Many Rohingya also report forced recruitment, growing insecurity and a lack of prospects, leaving dangerous sea crossings as one of the few perceived routes to safety.
(inputs from AP and BBC)
