Ministry of Defence (MoD) legal papers are feared to have been stolen from a lawyer on a train journey from Leeds.
The solicitor, from law firm Eversheds, told police a suitcase containing the documents went missing as she travelled to London on Monday morning.
She was working for the Treasury Solicitor's Department, which provides legal advice to government ministries.
It is understood the documents were not classified secret. British Transport Police is trying to trace the suitcase.
It is not known what case the papers relate to, but an Eversheds partner has been appointed to act for the MoD in the upcoming public inquiry into the death of Iraqi civilian Baha Mousa in British military custody in 2003. cctv
A spokeswoman for the Attorney General's Office said: "Legal papers in the possession of a lawyer from a firm of private sector solicitors working for the Treasury Solicitor's Department were taken from a train on 16 February.
"The possible theft is under investigation by the British Transport Police.
"Action is under way in attempt to recover the papers."
A British Transport Police spokesman said: "The passenger reported the suitcase as missing after she had travelled on the 9.05am train from Leeds to London King's Cross.
is being viewed and investigations are continuing to trace the suitcase."
No arrests have been made. THE BBC