A series of investigations into alleged sexual crimes committed by US marines in and around Darwin have been quietly dropped by Australian and American authorities.

That decision raises questions about whether Australia and the US are doing enough to investigate claims of sex crimes, and echoes decades of international concern about the cover-up of those crimes by US military personnel serving abroad.

ADFIS began an investigation into an allegation of inappropriate behaviour that stalled.

"All ADFIS action in this matter has ceased due to jurisdictional issues," the report stated.

... it was possible Australian and US forces were more concerned about the military relationship than they were about properly investigating the allegations: 'ADF are actively attempting to keep it out of the public eye'