By Amy Sawitta Lefevre and Panarat Thepgumpanat
BANGKOK (Reuters) - The resignation of a senior Thai policeman in charge of a sweeping human trafficking investigation raises serious questions over the country's commitment to end the illegal trade and protect its officers, a rights group said on Monday.
Major General Paween Pongsirin, whose team led arrests made since May, quit at the weekend saying an order to transfer him to Thailand's south would expose him to revenge by members of trafficking syndicates still at large.
Thailand has indicted 88 people suspected of involvement in human trafficking since launching a nationwide investigation into gangs following the discovery in May of 30 bodies in graves buried near the Thailand-Malaysian border, which prompted an international outcry.
One soldier and four police officers were among those arrested.
The clampdown triggered a regional migrant crisis when traffickers abandoned boats crammed with thousands of migrants at sea, rather than risk landing on Thai shores.
Despite that crackdown, the U.S. kept Thailand for a second year on Tier 3 - the lowest tier - in its annual Trafficking in Persons report in July for failing to comply with the minimum U.S. standards for the elimination of trafficking.
Sunai Phasuk, Thailand researcher at Human Rights Watch, said lack of protection for investigators and witnesses would impact the effectiveness of the trafficking trials.
National police chief Jakthip Chaijinda told reporters Praween could have asked for protection but chose to resign instead.
"This is a shame, Paween is someone with knowledge," he said.
Reuters could not reach Paween immediately.
On Tuesday, the 88 suspects will be moved from the south, where many of them were arrested, to Bangkok where a criminal court division set up in July specifically to handle trafficking cases will begin examining witnesses.
But the trials could prove lengthy and convictions are far from certain.
Investigations have been exacerbated by at least three cases of witness intimidation. A Reuters report in July underscored how key witnesses felt traffickers could target them, raising questions about the long-term effectiveness of the crackdown.
(Writing by Amy Sawitta Lefevre; Editing by Nick Macfie)