BANGKOK (Reuters) - Thai police said on Wednesday they were winding down an investigation into human trafficking gangs, and planned to send a report of their findings to the country’s attorney general by the end of June.

Thailand began a nationwide crackdown on human trafficking networks and suspected camps hidden deep in its jungles following last month’s discovery of more than 30 bodies buried in human trafficking camps in the southern province of Songkhla.

Police officials, politicians and a military general are among the 53 people to have been arrested in the largest such investigation by Thai police. Arrest warrants have been issued for nearly 90 suspects, with about 36 people still on the run.

“We think that the police side of the investigation will be wrapped up by the end of this month,” national police chief Somyot Pumpanmuang told reporters.

“We will send these cases to the attorney general by the end of June,” he added. “Those who haven’t yet been arrested can go straight to trial because we will hand over the names of those suspects too.”

Police in neighboring Malaysia announced the discovery last month of 139 graves in jungle camps used by suspected smugglers and traffickers.

More than 4,500 migrants have landed in Indonesia, Malaysia, Thailand, Myanmar and Bangladesh since Thailand launched its crackdown on people-smuggling networks. The United Nations estimates around 2,000 may still be adrift at sea.

In its 2014 Trafficking in Persons (TIP) report, the U.S. State Department downgraded Thailand to its ‘tier 3’ list of the world’s worst centers of human trafficking.

Critics argue that insufficient action by Thailand to stamp out human trafficking last year may result in a second year on the lowest tier.

Thailand identified fewer victims of human trafficking last year than in 2013 and convicted fewer perpetrators of the crime, according to a Thai government report.

The U.S. has been working with regional governments to help locate the thousands of migrants still at sea.