Royal Navy investigators flew to Belfast last week after a memory stick containing "restricted" information on naval manoeuvres and personnel around the UK was reported missing.
The Observer has learnt that two senior detectives from the Royal Navy Police's Special Investigation Branch met members of the Police Service of Northern Ireland to investigate the issue.
The detectives flew from Portsmouth to discuss the loss of a USB memory stick close to Belfast docks last month. The device was found at the Odyssey car park, near the river Lagan. It was offered for sale to a newspaper, which declined the offer. It was later handed to the police in Bangor. The device is understood to have contained 37 pages of information on Royal Navy personnel including name, ages and ranks.
It also contained "restricted" information on naval operations around the UK, and the whereabouts of Royal Navy officers. Security sources said this weekend that naval investigators would examine the device to see if any of the material was copied. They will also look for "electronic footprints" on the device to find out who last used it.
The Naval inquiry will focus on the location of HMS Hurworth, a warship, which was moored in Belfast Lough, close to the car park, where the device was found. The investigation will explore the possibility that personnel from the Hunt class mine counter-measure ship accidentally dropped the memory stick while away from the vessel.
At the end of last week, investigators flew back to Portsmouth to try to establish the last known official user of the device. They will board HMS Hurwurth at sea to conduct further investigations.
At the time of the security lapse, the Ministry of Defence said it was unaware of any memory stick lost in Northern Ireland.
Security sources said that, given the current level of threat from republican dissidents, any lapse was serious, as they have previously targeted not only police and troops but other members of the UK's armed forces.
