Detectives have resumed their search for a body after four people were arrested in a series of armed raids at sites allegedly linked to slavery.
More than 100 officers from Gwent Police and the Serious Organised Crime Agency (SOCA) targeted properties in south Wales after discovering a man living in poor conditions in Marshfield, Newport.
The 43-year-old man was found earlier this year - 13 years after he was reported missing by his family.
Forensic teams are now combing the site on the strength of intelligence which suggests the body of an unknown person may be buried there.
Monday's raids - at addresses in Cardiff, Penhow in Monmouthshire and Marshfield - came after police received further evidence to suggest people were being kept in poor conditions and forced to work for no pay.
A 66-year-old man and a woman, 42, both from Marshfield, a Cardiff man, 36, and a woman, 42, from Penhow, have been questioned in custody.
Detective Superintendent Paul Griffiths, who is leading the Gwent Police investigation - codenamed Operation Imperial - said: "Today's action is the latest phase of a long-running investigation into alleged offences of slavery and servitude.
"The investigation began when a 43-year-old man was found to be living at the Marshfield site in very poor conditions having been reported missing by his family for 13 years. Since then a specialist team of detectives has been investigating and gathering intelligence to ascertain whether this part of a larger criminal conspiracy.
"In addition to the suspected offences in relation to slavery and servitude, we are also acting on intelligence which suggests that the body of an unknown person may be buried at the Marshfield site."
A forensic archaeologist has been drafted in to help the search.