An elderly vulnerable man scammed out of more than £100,000 is just one of 15 victims of a gang.
The man from Hollingdean, Brighton, was stung after fraudsters visited his home three times in a year falsely claiming that drainage work needed to be carried out.
A total of 15 victims in Sussex have told officers they were called and pressured into handing over money for hire drainage equipment.
There have also been four other victims in Brighton, two each in Woodingdean and Eastbourne and one in each of Lewes, Southwick, Burgess Hill, Newick, Rottingdean and Hove.
Detective Constable Leon Ryan said: "We do not know if it is one gang responsible or several gangs but they all use the same tactics to get money.
"They target vulnerable members of the community and worry them by claiming drainage work needs doing in the area. They pressure them and ask for money to hire the necessary equipment.
"At times they have told victims if they don't get the work done it could damage their neighbour's home and try to make them feel guilty.
"It is very upsetting for the victims to discover they have handed over thousands of pounds for nothing. They feel embarrassed and foolish and there could be some victims who are so upset they have not contacted us.
"We need to speak to anyone who may have been a victim so we can track down those responsible and prevent more people being targeted."
There have also been a further six reports in Surrey and seven in London.
Of the 28 cases, 12 have involved the fraudsters being given money by the victims.
In some cases victims have been targeted repeatedly.
Anyone with information about the frauds should call 101, email firstname.lastname@example.org or call the independent charity Crimestoppers anonymously on 0800 555111.
If you are suspicious about someone who visits you at home call 999.