Discs containing information from three of the UK's most sensitive inquiries have gone missing after being put in the post.
The material relates to inquiries into the role of the police in the deaths of three men, Mark Duggan, Azelle Rodney and Robert Hamill.
Officials realised the discs were missing three weeks ago and one member of staff has since been suspended.
The government said it took the loss "extremely seriously".
The third is an inquest into the 1997 murder of Robert Hamill by loyalists in Portadown, Northern Ireland, which campaigners claim involved the collusion of police officers.
In each inquiry there were witnesses, including police officers, who were given anonymity because of possible threats to their safety.
Officials have not confirmed whether any of the missing documents included personal information relating to these witnesses.
The government said in a statement that it "takes information security extremely seriously, and this incident is a breach of the arrangements that should be in place."
It said "intensive searches" carried out with the help of police had failed to recover either of the two missing discs.
It specified there was no evidence to indicate that the loss was the result of "malicious intent", and that the Rodney, Hamill and Duggan families had been informed.
Mark Duggan's aunt Carole Duggan said the Ministry of Justice had "questions to answer".
The identity of the police marksman who killed Mr Duggan in north London in 2011, sparking nationwide riots, has never been made public.
She said: "The criminal justice system went out of its way to protect the identities of the officers who killed Mark, issuing all manner of reporting restrictions and anonymity orders in court. Now we learn that this apparently sensitive information has been 'lost in the post' at the Ministry of Justice."
Susan Alexander, the mother of Azelle Rodney said it was "shocking news and very disappointing".
She said: "My lawyers are in contact with the Ministry of Justice on my behalf to establish exactly what data from the case, and particularly that which relates personally to me, is on the discs that have gone missing."
The report on the Robert Hamill inquiry has yet to be published.
In a statement his family said: "We are at a loss to understand why any material relating to the Robert Hamill inquiry should have been posted or sent at this particular time, given the fact that the inquiry report itself was completed in February 2011."
Shadow Justice Secretary Sadiq Khan called the loss "an appalling lapse in security".
He said: "It would be disastrous if this data got into the wrong hands.
"The justice secretary needs to get an urgent grip on this situation and set out what the government is doing to find this data and reassure the public that measures are in place to prevent it happening again."
The Metropolitan Police, whose officers were involved in the Mark Duggan and Azelle Rodney cases, also emphasised that it was taking the data breach "very seriously".
The force disclosed it had "risk-assessed" the material and taken "appropriate" steps. It added it was offering support to the Ministry of Justice but was not conducting its own investigation.
A spokeswoman for the Information Commissioner's Office told the BBC: "We have recently been made aware of a possible data breach involving the Ministry of Justice.
"We will be making enquiries into the circumstances of the alleged breach before deciding what action, if any, needs to be taken."