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
Jonathan Theobald said: "The weather can change quickly and a car can become lethal, I've discovered that the hard way."
Political parties across the country suspended their General Election campaigns following the Manchester Arena terror attack on Monday.
Investigators believe Mr Kushner has significant information relevant to their inquiry, officials told NBC News.