The United States, which has censured Thailand for failing to act against human trafficking, called on Monday for a speedy and credible inquiry into the discovery of a mass grave containing more than two dozen bodies thought to be of ethnic migrants.

Police and volunteers exhumed 26 bodies on Friday and Saturday near a suspected trafficking camp deep in the southern Thai jungle near Malaysia that held as many as 400 trafficked migrants, mainly Muslim Rohingyas from Myanmar and Bangladesh reports a Thai daily, The Nation.

A spokesperson for the US State Department said the United States was in contact with the Thai government, the United Nations and the International Organization for Migration over the discovery and understood that Thai authorities were actively investigating reports of other camps.

"We encourage a transparent, credible and expeditious inquiry into this case," the official said. "We urge Thai authorities to investigate fully these deaths and camps, and prosecute those responsible."

On Monday, Thai police announced charges including human trafficking and holding people for ransom against a Rohingya man and three local government administrators. They said another four Thais were being sought.

The arrests, and the uncovering of the camp and the grave represent the first major disruption of a trade in humans that activists and some Thai officials say has been allowed to flourish for years amid indifference and, sometimes, complicity by Thai authorities.

The State Department official said it was not yet possible to determine whether authorities were complicit in the existence of the camp, or if this was a case of people smuggling or trafficking, but involvement by Thai officials in trafficking was a problem the United States had documented in the past.

Last June, the U.S. State Department downgraded Thailand to its lowest rank in a survey of countries’ efforts to eliminate human trafficking, placing it alongside states such as North Korea, Syria and Uzbekistan.

The U.S. official declined to say what ranking Thailand would be placed in the next survey due out in June.

A bottom-tier ranking exposes Thailand, a key US ally in Southeast Asia, to the possibility of sanctions in addition to those imposed since a military coup in the country last year, although these steps have so far been largely symbolic.