WANG KELIAN, Malaysia — Malaysian authorities said Monday a cluster of abandoned jungle camps used by human traffickers contained 139 suspected graves as well as barbed-wire pens likely used to cage migrants, shedding more light on a regional trade that preyed on some of Southeast Asia’s most desperate people.

National police chief Khalid Abu Bakar said forensics experts were exhuming the suspected graves found at 28 vacated camps in the hilly jungle area on the border with Thailand where trafficking syndicates were known to operate.

“It is a very sad scene,” Khalid told reporters at a police outpost in the town of Wang Kelian several kilometers (miles) from the camps, one of which appeared large enough to hold about 300 people. “I am shocked. We never expected this kind of cruelty.”

At one forest camp, police found several parts of a decomposed body inside a wooden pen. The parts were placed into white bags and brought to Wang Kelian, and district police chief Rizani Ismail said they would be examined by forensics experts. Police said they would begin digging up other suspected graves — mounds of earth, covered with leaves and marked by sticks — on Tuesday.

“We have discovered 139 of what we believe to be graves,” Khalid said. “We believe they are victims of human trafficking.”

Prime Minister Najib Razak, in Tokyo on an official visit, vowed to find the perpetrators. “I am deeply concerned with graves found on Malaysian soil purportedly connected to people smuggling. We will find those responsible,” he said on Twitter early Monday.

The finding in the northern Malaysian state of Perlis follows a similar discovery earlier this month by police in Thailand who unearthed dozens of bodies from shallow graves on the Thai side of the border. Thai police Maj. Gen. Puthichart Ekkachan said 36 bodies had been found there in seven abandoned camps.

The discoveries have exposed hidden networks of jungle camps run by human smugglers, who have for years held countless desperate people captive while extorting ransoms from their families. Most of those who have fallen victim to the trafficking networks are members of Myanmar’s persecuted Rohingya Muslim community or impoverished migrants from Bangladesh, part of a wave of people who have fled their homelands to reach countries like Malaysia, where they hope to find work or live freely.