Dogs at a vets and rescue centre were left in their own faeces in overcrowded kennels, a former employee has claimed.
The RSPCA removed more than 100 dogs from 4 Paws Veterinary Clinic, in South Killingholme, on Thursday after a warrant was executed by police.
Inspectors said they were investigating welfare conditions at the site in North Lincolnshire.
Following the raid, a number of people have spoken about their experiences with the kennels.
Speaking to BBC Look North, the former employee, who did not want to be named, said the dogs were kept in cramped conditions and were often covered in their own faeces when he went to attend to them in the morning.
A woman also claimed her dog came back from the kennels with an infection and was unable to sit down because of red-raw skin between his back legs.
One woman spoke out about the lack of support received when she had taken on a rescue dog from the centre.
Charlotte Morgan, who paid £235 for her dog Sherbet, said she was forced to take him back after he bit a friend.
She said she had repeatedly asked for help and advice on training for Sherbet but received no response.
Ch Insp Ian Briggs, from the RSPCA's special operations unit, said anyone who had obtained animals from 4 Paws or had dogs at its kennels to get in touch.
No arrests have been made but he said a number of people were "helping us with our inquiries and have been taken away by the police".
"We're investigating everything about this organisation," said Mr Briggs.