Authorities said 26 bodies have been recovered from the graves since the camp was discovered Friday
in Sadao district of Songkhla province -- an area near the Malaysian border considered a trafficking hub.
Police General Ek Angsananont said that three suspects -- two deputy village chiefs and a municipal official -- had been arrested, as well as a Rohingya man from Myanmar.
He told CNN the men were acting as camp managers and middlemen.
A Thai court had issued arrest warrants for four other suspects who were on the run, he said. The men at large were Rohingya, members of a stateless Muslim minority who have been fleeing ethnic violence in Buddhist-majority Myanmar for years.
Ek said that once the remains -- of 24 men, one woman and another of unidentifiable gender -- had been examined, officials believed they had died between three months to a year ago.
Initial forensic examinations had found no sign of death caused by force or violence, he said, and investigators believed that deaths were due to illness.
The advocacy group Human Rights Watch said in a statement Friday that the dead are Rohingya Muslims from Myanmar or Bangladesh. The group cited police reports that say the victims "starved to death or died of disease while being held by traffickers who were awaiting payment of ransoms."
Some of the remains were found in shallow graves, while others were found in the open, covered only by blankets or clothes, the HRW statement said.
"The finding of a mass grave at a trafficking camp sadly comes as little surprise," HRW Asia director Brad Adams said in the statement. "Trafficking of persons in Thailand has long been out of control, something that senior officials have admitted to HRW and others.
Last year, the U.S. State Department downgraded Thailand to the worst possible ranking -- tier 3 -- in its Trafficking in Persons (TIP) report
. It said Thailand is a source, a transit point and a destination for trafficking. The report also said that ethnic minorities and citizens of neighboring countries were especially at risk of exploitation in Thailand through forced labor or the sex trade.
The fate of Rohingya Muslims was an area of particular concern for the State Department. The 2014 TIP report
said the Rohingya are among those most vulnerable to trafficking. The Rohingya usually arrive in southwestern Thailand by boat and travel from there to Malaysia, where they often work illegally.
In January 2014, Thai police found hundreds of Rohingya men, women and children in a camp in the same province and near the same town where the bodies were discovered Friday.