The Pentagon has disclosed that it suffered one of its largest ever losses of sensitive data in March when 24,000 files were stolen in a cyber-attack by a foreign government.
William Lynn, the US deputy secretary of defence, said the data was taken from the computers of a corporate defence contractor.
He said the US government had a "pretty good idea" who was responsible but did not elaborate.
Many cyber-attacks in the past have been blamed on China or Russia, and one of the Pentagon's fears is that eventually a terrorist group will acquire the ability to steal data.
Mr Lynn disclosed the March attack in a speech outlining a new cyber-strategy, which formally declares cyberspace a new warfare domain, much like air, land and sea.