On Sunday, Malaysian authorities said they had discovered a series of mass graves in abandoned camps, used by human traffickers, along the country’s border with Thailand, the Associated Press reports. Police in Thailand made a similar discovery earlier this month.

The New York Times reports that more than 139 graves have been discovered—some containing more than one body—across 28 camps. Traffickers use such camps to hold Rohingya Muslims and other refugees fleeing Myanmar and Bangladesh captive, demanding ransoms from their families.

From the Times:

“When they initially get on the ship from Myanmar, they think $200 is the cost all the way to Malaysia,” said Amy Smith, an executive director of Fortify Rights, a human rights group focusing on Southeast Asia. “It’s only when you enter Thai waters and are brought to a remote island or jungle camp under armed guard” that the migrants are “told to call family members in Malaysia, call family in Myanmar.”

Migrants are usually forced to pay an additional $1,000 or $2,000 before they are released from the camps, Ms. Smith said. “People are being beaten while they are on the phone to ask for more money, to put pressure on family members,” she said. “This isn’t just extortion. People’s lives are at risk.”

According to the AP, there are more than 1.3 million Rohingya—“one of the most persecuted minorities in the world”—living in Myanmar. More than 140,000 have been displaced as a result of attacks there since 2012, included 100,000 who have fled by sea.

Thousands of refugees are currently believed to be trapped aboard boats abandoned by their captains, the AP reports.

Photo credit: AP Images. Contact the author of this post: brendan.oconnor@gawker.com.