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Malaysia has said it believes 139 people are buried in graves at remote detention camps used by people smugglers on the Thai border which were discovered on the weekend.

Deputy Home Minister Wan Junaidi Tuanku Jaafar said on Thursday that each grave site contained the remains of one person, not multiple bodies as previously feared.

"Based on the size of the graves, and after the area was cleared... we have a clearer indication, single grave, single person," Jaafar said, according to the AFP news agency.

"The bodies were wrapped in white cloth. It is like the Muslim burial ... some are shallow graves, not all." 

Northern Malaysia is on a route for smugglers bringing people to Southeast Asia by boat from Myanmar, most of them Rohingya Muslims who say they are fleeing persecution, and people from Bangladesh seeking work. Smugglers have also used southern Thailand.

Malaysian police exhume bodies from mass grave

Al Jazeera's Florence Looi visited migrant camps in Perlis state in Malaysia this week, including one camp where 37 graves were found. She said the camps in the area may have housed hundreds of people.

"Authorities say they believe this camp was abandoned several years ago. Much of the structure is in ruins, but you can still tell it was probably big enough to hold about 200 people, " Looi said.

"All around the camp is traces of human activity...plates, crockery, basins thrown into a pit, pieces of cloth that may have been curtains and crudely built cages where trafficking syndicates kept their human cargo."

The camp Looi visited is on a hill, less than 500 metres from the Thai border.

Around 28 human trafficking camps were found by the Malaysian police over the weekend, less than a month after Thai authorities made a similar discovery in their territory.

Malaysian authorities confirmed to Al Jazeera on Thursday that 12 police officials have been arrested over the course of the past year on suspicion of links to people-smuggling camps.

Source: Al Jazeera and agencies