As the number of desperate Rohingya and Bangladeshis shuttled through the trafficking operation shot up, so did Manas’ rank in the Thai military.
But the silver-haired general was condemned to 27 years in prison on July 20 for profiting from the trade, an extraordinarily rare conviction of a senior member of an army that dominates the kingdom.
The 61-year-old’s downfall was hastened in 2015 after investigators uncovered secret jungle prisons in the south where traffickers starved and tortured migrants while holding them for ransom.
The discovery exposed Thailand’s horrifying role in a criminal operation that shifted victims from Myanmar to Malaysia, and forced the ruling junta to launch a belated crackdown.
Police followed a money trail that lead straight to Manas, an army hard-liner with a passion for bullfighting.
“He was involved in such an obvious way...at a time when the junta was really trying to show themselves to be clean,” said Paul Chambers, an expert on Thailand’s military.
“He is going down because he was at the wrong place at the wrong time.”
Manas was first highlighted as a suspect in early 2015 after 98 famished Rohingya were found in trucks in Nakhon Sri Thammarat, stopped by a random police checkpoint.
Provincial police – aided by anti-trafficking NGO Freeland – used the drivers’ cell phones to trace their regular route.
The trail carved through Thailand’s southern neck from coastal Ranong, where boatloads of migrants arrived from Myanmar, to malaria-infested camps near the Malaysian border, where they were held in appalling conditions.
Phone and e-banking records from the drivers led to key trafficker Sunan Saengthong, a Ranong politician and businessman who had deposited nearly $600,000 (B19.98 million) in accounts belonging to Manas.
In May 2015 police found more bank slips revealing that Sunan’s nephew had also transferred huge sums to Manas, including some $400,000 (B13.33mn) in just over a month.
Sunan was jailed for 35 years in a separate trial but his nephew Nattaphat Saengthong and others remain at large.
Around the time of the money transfers, Manas served as a top commander of Thailand’s southern security arm.
His job was to enforce its controversial “push-back” policy – which meant turning around boats of stateless Rohingya who were trying to flee persecution in Myanmar.
But he used this position to do just the opposite, according to the July 20 verdict, which exposed a matrix of collusion between state officials and businessmen who profited from trafficking.
Witnesses said Manas instructed officers to force back a boat of 265 Rohingya in 2012 – only to covertly re-route the ship to shore and truck the human cargo south to the jungle prisons.
Manas “had direct responsibility in the push-back mission and must have been part of this human trafficking network, otherwise the Rohingya would not have been able to return to Thailand so quickly,” the verdict read.
The trafficking operation flourished until the 2015 crackdown, with tens of thousands of victims funnelled through a trade worth an estimated $250 million dollars (B8.33 billion).
Many were lured from the Myanmar-Bangladesh border by brokers who promised jobs, while others were violently kidnapped and forced onto the boats.
The big money was made in Thailand, where jungle camp wardens phoned relatives of the weakest migrants and threatened to kill them if they didn’t send more cash.
The young and strong were sold off as labour to Malaysian palm oil plantations or fishing boats, according to Freeland.
All the while, Manas’ seemingly inexorable rise up the army ranks continued, with his command stretching over increasingly large chunks of the south.
Months before his arrest in 2015, he was promoted to Lt Gen and given the sweeping role of “military advisor”.
It wasn’t the first time the hawkish officer had hurdled controversy.
He was linked to a 2004 raid on a mosque that left more than 30 Muslim rebels dead in Thailand’s far south, one of the early sparks of an insurgency still burning today.
“He had a reputation for often going beyond the law,” said Chambers, adding that he was known as a “big shot” in the region.
Manas was the only military man convicted in the recent trafficking trial, which saw more than 60 people sent to jail.
Rights groups welcomed the verdict but warned that many perpetrators remain at large.
“We know not everyone has been accounted for in this trial,” said Amy Smith from Fortify Rights, which closely tracked the investigation.
“More needs to be done to account for the horrific crimes that took place... and to ensure this never happens again.”