Where doubt lay in the cause for an aircraft loss or it was noted that a trend of losses was occurring then the Accident Investigation Branch could be tasked with investigation by the Air Ministry.
Pre-war the AiB had mostly been concerned with civil air losses but as a part of the Air Ministry it also had a watching brief for military air losses. During the war period it was mainly concerned with military crashes needing more investigative skill than a Court of Inquiry could provide.
Later Form 765C, Form 1180 and Form 412 all have a section where it was noted that an AiB Investigation was taking place into the accident.
Not all AiB reports have survived, most of 1944 is missing, but those that were saved are stored at The National Archives, Kew, London under the civil AVIA 5/Series.
Typically of 3 to 4 pages they can contain photos of recovered wreckage as well as witness statements.