The Serious Fraud Office (SFO) said that instead of returning thousands of pages of evidence to 59 different sources, it sent the entire batch of material to a single recipient.
The office said it had already contacted the 59 people involved and that it is "working to contact any others who may have been affected".
More top news
Penny Wilson says her mother Sylvia Giffin was bullied after voicing her concerns.
The call came on the 70th anniversary of the arrival of the Empire Windrush.
Rentokil said callouts to moth infestations were up by 60% over the last four years as consumers reported they were buying better-quality cl