A man believed to be a soldier has been released without charge after being detained and questioned about a breach of security at the Castlereagh police complex in Belfast.
The investigation, which is believed to be concentrated in a part of the building used by the British Army, is ongoing.
A spokesman for the Police Service of Northern Ireland said the incident did not involve police security and was not thought to involve a break-in. A spokesman for army headquarters in Northern Ireland confirmed that the case related to its premises in the complex.
He said: "We can confirm that the military are assisting the PSNI in an investigation into a possible internal breach of security at military premises in the PSNI Castlereagh complex."
A break-in at Special Branch offices at the same Castlereagh base in March 2002 created a major security alert into what documents might have been seen by paramilitaries.
The IRA was blamed and it put the devolved Stormont administration under pressure when unionists demanded sanctions against Sinn Féin.