Slavery farm inquiry 'shocking', says Home Office
As police continue to search for human remains at a farm, the Home Office says the case serves as an "appalling reminder of the extent to which slavery has reappeared in our country".
Officers are using mechanical diggers to excavate a site between Cardiff and Newport.
Four people are being questioned following raids by an anti-slavery Operation Imperial.
On Monday, a second man was found living in poor condition on the site.
The multi-agency inquiry was sparked earlier in the year by the discovery of a "vulnerable" man found living in poor conditions on Cariad Farm in Peterstone - described in a Newport Council document as gypsy and traveller family-owned accommodation.
Darrell Simester, 43, from Kidderminster, Worcestershire, had been missing for 13 years since disappearing while on a holiday in Porthcawl, south Wales.
On Monday police confirmed that another man, believed to be from Poland, had been taken to a "place of safety". His mental and physical health are being assessed by the Red Cross.
The farm remained sealed off after a team of more than 100 police officers cordoned it off along with two properties in Cardiff and Monmouthshire early on Monday.
But police said on Tuesday that Cariad Farm was now the only focus of their investigations.
Detectives also said an anonymous letter sent to officers could be crucial to the inquiry.
Senior investigating officer Det Supt Paul Griffiths said: "I'm making a personal appeal to the author of an anonymous letter which was sent to a neighbouring force some months ago to contact us.
"The letter contains information which may be vital to this investigation."
The four arrested - a 66-year-old man and a 42-year-old both from the Marshfield area, a 36-year-old man from the Cardiff area, and a 42-year-old woman from Penhow - remain in police custody.
Det Supt Griffiths added: "As a result of the publicity generated by the police activity I'm grateful to the people who have contacted us with information which is assisting our inquiry.
"Importantly, we've received a number of calls from individuals who may also be potential victims or important witnesses."
He also thanked the community for their understanding while inquiries continued.
On Monday, Ch Insp Huw Nicholas said the raids had followed six months of investigation and that efforts would continue to establish whether a body had been buried on the farm.
"We do have information there may be a body buried at that site and it would be remiss of us at this time not to investigate those fully, hence the scale of our operations there," he said.
"We will exhaustively inquire into that but at the moment we have no more than the suggestion that there could be a body buried there.
"We anticipate being there for some three days."
A large manure and compost heap is being examined, and officers are being assisted by a forensic archaeologist.
There are other residents on the farm land and anyone entering or leaving is being asked to sign in and out.
Operation Imperial 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.
Gwent Police is working in conjunction with the Serious Organised Crime Agency (SOCA), other forces, the UK Human Trafficking Centre, RSPCA and the Red Cross.
The force says the potential offences they are investigating are serious, and officers have appealed for anyone with information to contact them.
A Home Office spokesperson said: "The home secretary has made clear her determination to tackle the scourge of modern slavery and the organised criminal gangs behind it.
"We are already taking action on a number of fronts. We are working with frontline professionals to help them better identify and support potential victims.
"The new National Crime Agency will, from next month, lead an enhanced and co-ordinated response to targeting trafficking gangs, and we will be overhauling and updating legislation by bringing forward a draft Modern Slavery Bill this session."