* Roti seller's story leads to big break in investigation

* Police allege official indifference to investigation.

* Fears for the safety of key witnesses

* Lawyers for key suspects from pro-establishment Democrat Party

* Experts warn "big fish" at large, investigation should widen

Sheltering in the backroom of a provincial Thai police station is a 35-year-old street vendor who triggered a human trafficking investigation that has reverberated across Southeast Asia.

He is a Rohingya Muslim, a mostly stateless group from western Myanmar. He had scraped a living for the past decade selling fried bread, or roti, from a push cart in Nakhon Si Thammarat, a city in southern Thailand.

Then his nephew fell into the hands of murderous human traffickers.

The roti seller's desperate bid to save him ultimately led to the discovery of scores of jungle graves on the Thai-Malaysia border in May and sparked a regional crisis over boatloads of unwanted Rohingya.

Now the roti seller fears traffickers could target him. His new home in the police station is a primitive form of witness protection. (Reuters has withheld his identity at the request of police.)

His predicament raises questions about the long-term effectiveness of Thailand's crackdown on resilient and lucrative trafficking syndicates. Witnesses have been intimidated, police say. Key suspects are represented by lawyers with powerful political connections. And while 72 people have been arrested, police are still seeking many others.

Thailand's investigation comes ahead of a new U.S. report card on its anti-trafficking efforts, due out in mid-July. Police spearheading the campaign on the ground told Reuters they encountered official indifference about the evidence they had gathered on trafficking networks - even after the U.S. State Department identified Thailand in June 2014 as one of the world's worst trafficking offenders.

Katrina Adams, a spokeswoman for the State Department's East Asia and Pacific Bureau, said this year's report only covers the year to March 2015, and thus would not include Thailand's latest crackdown.

"We welcome Thailand's law enforcement actions, including the arrests of dozens believed to be involved in migrant smuggling and abuses against migrants, which may include human trafficking, in southern Thailand," Adams added.

INDIFFERENCE

Police Major General Thatchai Pitaneelaboot, who led early anti-trafficking efforts in southern Thailand was told his investigation was damaging Thailand's image, though he declined to be more specific about who was telling him that.

"No one cared," he said.

Thatchai felt otherwise. "If we want to eradicate human trafficking, we can't hide it. We must put it on the table."

Deputy National Police Chief Aek Angsannanont, who is in charge of the anti-trafficking crackdown in Thailand, said the military government that came to power in a coup last May took the issue seriously.

"I don't know what the policy was of previous administrations," Aek said. "I took up this trafficking issue under the military government and the military government has given this issue importance."

After last year's coup, Thailand's military junta promised what it called a "zero tolerance" policy to human trafficking. Yet Thailand convicted fewer perpetrators of human trafficking last year than in 2013, according to the government's own anti-trafficking report.

Aek said he could not "give an opinion on this. But I can say that since the June 2014 (U.S. anti-trafficking) report, everyone woke up and has taken this issue seriously."

The Thai crackdown has disrupted the region's trafficking infrastructure for now but some experts question how lasting that will be.

The investigation has "made trafficking in Thailand a bit harder," said Steve Galster, director of FREELAND Foundation, an anti-trafficking NGO that has given technical help to the Thai police. "The question remains, however, if anyone higher up the chain . . . will be investigated." If that doesn't happen, Galster warned, "trafficking in this region will remain a big problem."

PREYING ON ROHINGYA

The trafficking syndicates have particularly preyed on the Rohingya, who are fleeing poverty and oppression in Myanmar. The number of people leaving on boats from Myanmar and Bangladesh has nearly tripled in three years - from 21,000 in 2012 to 58,000 last year, according to The Arakan Project, a Rohingya advocacy group based in Bangkok. Most of them came ashore in Thailand and were moved to trafficking camps.

The camps along the jungly border between Thailand and Malaysia had been exposed as early as 2013. But they became impossible to ignore in May after police from both countries found the graves of 175 suspected migrants at dozens of hastily vacated trafficking camps on both sides of the border.

The ensuing crackdown meant traffickers could no longer bring their human cargoes ashore so they simply abandoned them at sea. The boats eventually washed ashore in Malaysia, Indonesia and Myanmar, their passengers sick and thirsty. At least 1,200 remained stranded at sea, according to a June 16 United Nations report.

The roti seller, who Reuters interviewed at the police station, said his nephew fell into the hands of traffickers during last year's smuggling season.

Last October, he said his family paid 95,000 baht ($2,800) in ransom money to free their 25-year-old nephew from a camp in southern Thailand. Traffickers typically held boat people for ransom and often tortured them until their relatives, who had settled in Thailand or Malaysia, paid up. Some of those whose relatives couldn't pay were left to die in the camps. Police say some were sold into slavery on Thai fishing boats.

Despite getting the ransom payment, the roti seller said the alleged operator of the camp his nephew was in, a Myanmar man known as Anwar, refused to release his nephew. It was unclear to him why.

So, two months later in December, the roti seller filed a complaint against Anwar with local police. "They didn't take me seriously," he said.

Police Colonel Anuchon Chamat, deputy commander of Nakhon Si Thammarat Provincial Police, admitted they were "not that interested" in the complaint at the time.

That was about to change.

TRANSPORTATION NETWORK

On Jan. 11, just before dawn, Anuchon's men intercepted five trucks at a routine checkpoint in Nakhon Si Thammarat. Hidden inside were 98 tired and malnourished Rohingya. One woman had suffocated to death; two more later died in hospital.

Police interviews with the survivors confirmed what the roti seller had described: "That there was buying and selling of humans," Anuchon said.

He said he sought help from the anti-trafficking group FREELAND, which analysed data from mobile phones seized from two of the truck drivers.

This helped Anuchon map out a transportation network that led from Ranong, a port city on the Andaman Sea, to jungle camps on the Malaysian border, an overnight's drive away. He concluded that the malnourished Rohingya and the roti seller's nephew were in thrall to the same syndicate. Bank transfer slips from the roti seller showed he had paid the money to suspected syndicate members.

Anuchon's discovery, however, was too late to save the roti seller's nephew.

On Jan. 27, camp guards called the roti seller and placed a phone to his nephew's face. The roti seller wept as he described what happened next. The traffickers, he said, had found out he had gone to the authorities. Anuchon confirmed the roti seller's story.

"They're going to kill me," his nephew said. "What did you do?"

The roti seller heard the phone drop and his nephew screaming. Then a voice said, "He's dead already", and the line was cut.

INACTIVE INTELLIGENCE UNIT

Still, Anuchon did not think he had enough evidence to convince his superiors about the growing scale and sophistication of the trafficking networks. "We did not dare talk to Bangkok because our evidence was insufficient. If our information was wrong, we would have lost face with our bosses."

Yet one Thai police unit was well-placed to help monitor the Ranong-based syndicates, including the one that had held the roti seller's nephew captive. The Port Intelligence Unit in Ranong was set up in 2013, with help from the United Nations Office on Drugs and Crime (UNODC), to gather intelligence on people smuggling, human trafficking and transnational crime. But, lacking the go-ahead from Bangkok, it remained inactive.

The unit is "the right solution in the right place," said Jeremy Douglas, the UNODC's Regional Representative in Southeast Asia and the Pacific. "It has not become fully operational and it needs a (Thai) leadership push to get going."

Aek, the deputy national police chief, declined to comment about the status of the unit.

SHALLOW GRAVES

After intercepting the truck convoy, Col. Anuchon enlisted the roti seller's help in tracking down a Rohingya witness who had survived 10 months at the same camp as the nephew. At the request of police, Reuters has agreed not to reveal the survivor's name for safety reasons.

The Rohingya survivor said Anwar, the alleged camp operator, had ordered the nephew killed. On April 28, police grabbed Anwar after staking out his house and took him to Nakhon Si Thammarat's main police station.

The roti seller was already at the station, where earlier that day he had recounted how he had tried to tell police in four different cities about his nephew's plight. Anwar, flanked by policemen, walked past him in a corridor. "I wanted to hit him for what he did to my nephew," the roti seller said.

Anwar, 40, also known as Soe Naing from Myanmar's Rakhine State, is himself a Rohingya. During an hour-long interview at the police station, Anwar insisted he was not a human trafficker, but a rubber tapper - and a roti seller himself.

"They say I killed. I am not worried. I did not do anything and I don't know anything about this," Anwar said. "I'm rich enough selling roti."

Three days after Anwar's arrest, the Rohingya survivor led police to the camp a few hundred metres from the Malaysian border on a hill local people called Khao Kaew or "Glass Mountain." Police believed it had been hurriedly evacuated just days before. They discovered shallow graves marked with bamboo sticks.

A sombre mood descended as police and rescue volunteers unearthed 26 corpses on May 1. Some were shrouded in cloth or simple bamboo mats. Others were little more than skeletons.

When asked if there were more graves yet to be discovered along Thailand's border, Police Maj. Gen. Thatchai replied: "Absolutely."

ESTABLISHMENT LAWYERS

After Anwar came other big-name arrests. Patchuban Angchotipan - a wealthy businessman from Satun province known as Ko Tor or "Big Brother Tor" - gave himself up at a Bangkok police station on May 18. Patchuban, the former chairman of Satun's provincial administration, has been charged with a range of offences, including human trafficking, holding people for ransom and detention leading to bodily harm.

Patchuban was unavailable for comment. Fighting his case in court will be Wirat Kalayasiri, the chief legal advisor of Thailand's Democrat Party, which has close links to the military and royalist establishment. Wirat is also representing another key suspect, Anas Hajeemasae, who police describe as Patchuban's right hand man.

Pakkapon Sirirat, another Democrat Party member, is representing Lieutenant General Manus Kongpan, who surrendered to police on June 2. "I'm a lawyer and I have the right to be a member of a political party," Pakkapon said. "My job as a lawyer is to look after the accused."

Manus denies all charges, which include human trafficking, holding people for ransom and hiding corpses.

Manus previously headed an operation to intercept migrants in the Andaman Sea for the Internal Security Operations Command, Thailand's powerful, military-run equivalent to the U.S. Department of Homeland Security. "If Manas really is involved in trafficking, he won't escape it and will have to accept the truth," Pakkapon said.

The trials could be lengthy and convictions are far from certain, police said.

On June 16, three men were arrested for intimidating a witness not to testify in the trials. Other witnesses have been threatened by "subordinates" of the accused against testifying, said Aek, the deputy national police chief. "The suspects are powerful people," he said.

HUNDREDS INVOLVED

Moreover, the scores of arrests so far may only represent a fraction of those involved, police say. "There could be hundreds of people involved, including many officials," Thatchai said.

And despite the investigation and crackdown that began in late April, the traffickers' finances seem largely intact. The United Nations estimates people-smuggling across the Bay of Bengal has generated about $250 million since 2012. Thailand has so far seized assets worth only $3.5 million.

Aek said Thai authorities "only froze assets of those we suspected of wrong-doing".

The roti seller dares not leave his new home in the provincial police station. He recently stopped praying at a nearby mosque after he heard that some men had turned up to look for him there.

Many known traffickers remained at large, which was why he hoped to be relocated to another country after the trial. "Otherwise," he said, "I will be killed."