More than 11,000 people have been abducted in 20 years of slave-raiding in Sudan, a new report says.
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Some 10,000 of these are still missing and many are being held as slaves, one of the report's authors told BBC News Online.
The East Africa and United Kingdom-based Rift Valley Institute released its report on the basis of thousands of interviews in the Northern Bahr-el-Ghazal province, which it says is worst affected.
The information will be used by human rights groups to help trace the abductees and reunite them with their families.
John Ryle told BBC News Online that many of them were being held by northern, Arab militias.
His co-director, Dr Jok Madut, told the BBC's Network Africa that he was surprised to find that the majority of the abductees were young men.
528 returned home
5,148 killed in raids
Source: Rift Valley Institute
"60% of them are young men who were abducted from cattle camps where they were herding livestock," he said.
He said they were often abducted when they tried to stop the raiders stealing their cattle.
"It's a reasonable assumption that many of them were taken into captivity in the north," Mr Ryle said.
However, some will have ended up in camps for internally displaced persons, he said.
The researchers covered the vast region by bicycle and on foot, carrying questionnaires, clip-boards and mosquito nets.
They recorded detailed information on each missing person, including their age, sex, village, clan affiliation, the circumstances of their abduction and the names of their surviving relatives
"This has given us a clear picture of the scale of abduction. We are using it to verify information and facilitate the retrieval of those still missing", says Peter Hawkins, Regional Director of Save the Children Fund (UK).
"Abduction and slavery are horrific; the important thing is that we now know what the facts are. We know for certain who has been abducted, how many, where and when," Dr Madut said.
In the worst affected village, Ajok in Aweil West County, 101 adults and children were abducted in a single week, the report says.
Simon Wol Mawien, a local administrator in Aweil County told the researchers that the research was a critical tool for cultural survival and peace-building.
"Our children have been stolen from us. We trust these lists will help to get them back," he said.