Police investigating alleged slavery at a farm have appealed for the writer of an anonymous letter to come forward.
Officers have resumed their search of a property in the Marshfield area of South Wales as part of their inquiries - that also includes digging for a body of an "unknown person" which may have been buried at the site.
The senior officer leading the probe - named Operational Imperial - is urging people with information on the case to contact police.
Four people have been arrested in connection with the investigation so far.
A 66-year-old man and a 42-year-old man, both from the Marshfield area; a 36-year-old man from the Cardiff area; and a 42-year-old woman from Penhow were detained on suspicion ofslavery and servitude offences.
Gwent Police's Detective Superintendent Paul Griffiths said the force had received a number of calls from individuals who could be potential victims or important witnesses.
He said: "I'm urging those individuals to contact the investigation team again by ringing our dedicated number 01633 647174 as we believe they may have more information which could be of assistance.
"Any information can be provided in complete confidence and officers and staff can provide any appropriate support.
"I would also urge anyone else who may have any other information relating to this investigation, no matter how unimportant it may seem, to pick up the phone and contact us.
"In addition, 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 and we need that person to contact us."
Police said their investigation at the farm was sparked after the shock discovery of a man in Marshfield who had been missing for 13 years.
Darrell Simester, of Worcestershire, had torn clothes and teeth missing when he was found by police last March.
It has been reported in the Daily Mail that Mr Simester told police about an alleged slavery gang in Marshfield.
The newspaper's website said the 43-year-old spoke of being forced to work 12 hours a day and was too scared to try to escape.
Superintendent Griffiths added: "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 is 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.
"As such, specially trained officers, equipment and a forensic archaeologist have been deployed to investigate this."
Neighbours in Marshfield have spoken of their shock at the police probe, which saw police set up a cordon outside the entrance to the farm and a number of specialist teams arrive at the scene, including a scientific investigation unit.
One neighbour, who wanted to remain anonymous, said: "It's just shocking. I just cannot get my head around this. I don't think I've ever seen so many police in my life."
Police said they were aware their operations had caused disruption to residents, but were grateful for their continued cooperation and understanding.
"Local officers are in the area to provide reassurance and if anyone has any concerns I would urge them to speak to these officers," Superintendent Griffiths said.
The force said it was also working in partnership with with other forces, the Serious Organised Crime Agency (SOCA), UK Human Trafficking Centre, RSPCA and the Red Cross.
Veterinary surgeons are expected to be at the scene today to check on the wellbeing of horses on the farmland.
The Home Office described the case as "shocking".
A spokesman said: "It is an appalling reminder of the extent to which slavery has reappeared in our country.
"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 front line professionals to help them better identify and support potential victims.
"The new National Crime Agency will, from next month, lead an enhanced and coordinated response to targeting trafficking gangs, and we will be overhauling and updating legislation by bringing forward a draft Modern Slavery Bill this session."