Four people have been arrested in a series of major armed police raids involving more than 100 officers at addresses allegedly linked to slavery.
Gwent Police and the Serious Organised Crime Agency (Soca) targeted addresses in Cardiff, Penhow in Monmouthshire and Marshfield, Newport, in south Wales.
A 66-year-old man and a woman, 42, both from Marshfield, a Cardiff man, 36, and a woman, 42, from Penhow, all remain in custody assisting police with enquiries. An east European man was also taken from the Marshfield address to hospital for assessment of his physical and mental condition.
Forensic teams are also searching the Marshfield site on the strength of "intelligence" suggesting the body of an unknown person may be buried there. The raids were triggered by the discovery of another man living in poor conditions at the Marshfield location earlier this year.
The Gwent Police investigation, codenamed Operation Imperial, has seen the force team up with Soca, the UK Human Trafficking Centre, the RSPCA and the Red Cross.
"So far, uniform officers and detectives from Gwent Police, working with other specialist units including members of the dog section, air support unit, the armed response unit have entered premises in relation to offences regarding slavery and servitude from the Criminal Law Act 1977 and the Coroners and Justice Act 2009," a police spokesman said.
"Four people have so far been arrested as part of this ongoing investigation.
"A dedicated team of detectives has been responsible for bringing the long-running investigation to this latest phase which has centred around evidence and intelligence suggesting that people were being kept in poor conditions at the Marshfield site and forced to work for no pay.
"A man has been taken from the Marshfield site to an undisclosed medical reception centre where his physical and mental condition can be assessed and by specialist staff. The man whose age is unknown at this time is of Eastern European origin.
Detective Superintendent Paul Griffiths, who is leading the investigation, said: "The offences that are being investigated are extremely serious and we would urge anyone with any information which could assist the team to contact Gwent Police on 101 quoting log 66 23/09/13. If someone has information and would rather not pass this directly to the police then I would urge them to contact Crimestoppers, which is independent of the police and guarantees complete anonymity, on 0800 555 111."