WANG KELIAN, Malaysia (AP) — Malaysian authorities said Monday that abandoned jungle camps used by human traffickers contained over 100 suspected graves as well as pens likely used as cages for 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 139 suspected graves were found amid signs of "cruelty and torture" at 28 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 in the northern state of Perlis at a police outpost several kilometers (miles) from the camps.

At one camp, police found "a highly decomposed body" that will be examined by forensics experts as teams began the work of digging up the areas believed to be graves — mounds of earth, covered with leaves and marked by sticks, Khalid said.

"We have discovered 139 of what we believe to be graves," he said. "We believe they are victims of human trafficking."

The finding 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. 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 impoverished refugees fleeing Myanmar and 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 free from persecution.

As Southeast Asian governments have launched crackdowns in recent weeks amid intensified international pressure and media scrutiny, traffickers have abandoned camps on land and boats at sea to avoid arrest.

Khalid said that at least two of the camps appeared to have been abandoned within the past few weeks, based on the condition of items left behind such as vegetables, rice and almost new cooking utensils. The camps ranged in size, with the largest capable of holding about 300 people and smaller ones for just a few dozen.

On three large whiteboards, police pasted dozens of pictures taken at the camps. Some showed large, crudely built pens made from wooden sticks, which were believed to be used as cages.

"We think the migrants were imprisoned in these wooden pens," Khalid said. "They were not allowed to move freely and traffickers kept watch at sentry posts."

Other items left behind were a rifle pouch, used bullets and bullet casings, he said. Police also found white muslin cloth, used by Muslims in Malaysia to wrap dead bodies, and a wooden stretcher believed to have carried bodies.

The findings were the result of an operation Malaysian authorities conducted from May 11-23, during which they combed the steep jungle area along 50 kilometers (30 miles) of the border with Thailand. Due to the difficult terrain, Khalid said the investigation and forensics analysis could take a few weeks.

Human rights groups and activists say the area on the Thai-Malaysia border has been used for years to smuggle migrants and refugees, including Rohingya Muslims, a persecuted minority in Myanmar.

In many cases, they pay human smugglers thousands of dollars for passage, but are instead held for weeks or months, while traffickers extort more money from families back home. Rights groups say some have been beaten to death, and The Associated Press has documented other cases in which people have been enslaved on fishing boats.

Since May 10 alone, more than 3,600 people — about half of them from Bangladesh and half Rohingya from Myanmar — have landed ashore in Indonesia, Malaysia and Thailand. Thousands more are believed to be trapped at sea in boats abandoned by their captains.

Malaysia and Indonesia announced last week that they would provide temporary shelter for up to one year for migrants recently found or still stranded at sea. The U.S. has said it will settle some of them permanently.

The Rohingya, numbering around 1.3 million in Myanmar, have been called one of the most persecuted minorities in the world. Long denied basic rights, they have been driven from their homes in mob attacks in Myanmar's Rakhine state several times since 2012.

More than 140,000 were displaced and are now living under apartheid-like conditions in crowded camps. More than 100,000 others have fled by sea.

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Associated Press writers Jocelyn Gecker and Todd Pitman in Bangkok contributed to this report.