It may taint the witness testimony. However what I would do, is offer full immunity to the first 100 people innext 3 days that come foward with verified proof of the fraud. Then after 3 days, nake arrests of anyone we know commited fraud, just 2 or 3 low level people, then give 35 people 2 days to come forward with full immunity, then arrest a few others, and then final chance of 15 people in next 36 hours.

That would shake up enough to get some people to panic and come foward.