Thailand has long been a major hub for human trafficking and people smuggling, with rights groups accusing officials of turning a blind eye to the multi-million dollar industry - and even complicity in it.
Major General Paween Pongsirin headed a police investigation into trafficking after dozens of shallow graves were uncovered on the Thai-Malaysia border.
They were the sites of trafficking camps where victims - mainly from Myanmar's Muslim Rohingya minority and Bangladesh - were held in appalling conditions, many awaiting ransom payments from relatives for their release.
"I feared for my life because of the human trafficking cases. During my work... I was threatened and it became worse and worse."
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Paween resigned from the police rather than take up a new post after allegedly receiving death threats, he added.
"No one can protect me now. There is no sympathy, or mercy, for me from my bosses," he said.
The kingdom's pivotal role in the trade emerged in May as people-smugglers abandoned thousands of migrants at sea or in jungle camps after a Thai crackdown.
Dozens of people including Manas Kongpan, a powerful Thai army general in the south, and other local officials appeared in court last month charged with trafficking.
The police case against the 88 accused pivots on bank transfers of hundreds of thousands of dollars from known traffickers.
Paween said the arrests were made after his probe had found regular payments of up to USD 380,000 to the accounts of a number of key officials.
Manas is accused of being a linchpin, using his local influence to funnel migrants through the south, though he denies the charges.