It's feared the secret identity of police marksman V53, who shot Mark Duggan dead in Tottenham, could now be in the public domain after a file containing the details was allegedly "lost in the post."
The Ministry of Justice, the Information Commissioner and the Met Police are all investigating the possible data breach.
Mr Duggan was shot twice in August 2011. His killing that sparked a riot in Tottenham, across London and later across the country.
The inquest into his death found that the police marksman, V53 had acted lawfully, despite Mr Duggan being unarmed.
A Met police statement reads: "We have carried out a full risk assessment on material that we have been informed was on the disk and have taken the necessary steps. Due to the nature of the material we are not prepared to discuss what those steps entail."
The names of police firearms officers and others involved in sensitive cases are kept secret in case of those who might want to enact revenge. It is thought to be especially sensitive information in a case as controversial as that of Mark Duggan case, where much local tension remains.