Among the debris of wood, bamboo and plastic tarpaulin at an abandoned camp in the dense jungle here lies a coffin-size cage made from sticks tied together with rusted barbed wire. Next to it, an enclosure that could have held scores of people also bristles with barbed wire. And over a ridge, the police have started unearthing bodies from shallow graves.

The police in Malaysia have said little about the grim discoveries at desolate camps like this one on a hillside near the country’s border with Thailand. But what was left behind here suggests that the camps were busy holding stations for migrants under the control of ruthless, sophisticated human smuggling rings.

The Malaysian government on Tuesday took reporters on a two-hour trek through a buzzing jungle to view this camp in the state of Perlis, in the country’s far north. It is one of several where investigators say they have identified a total of 139 graves believed to hold the bodies of migrants, including members of the Rohingya minority fleeing religious persecution in Myanmar.

… The smuggling rings demand payments as high as $3,000 to release their clients, who are sometimes starved and abused to put pressure on relatives to come up with the money, according to Alistair D. B. Cook, a research fellow at the S. Rajaratnam School of International Studies at Nanyang Technological University in Singapore.

“Extortion isn’t the word for it,” Mr. Cook said. “The alternative is death. It’s worse than extortion.”