evidence: US prosecutors have obtained an audio recording of Donald Trump in which he acknowledges keeping a classified document after leaving the White House. [...] The justice department is investigating Mr Trump's handling of classified material. He denies wrongdoing. [...] The tape appears to contradict Mr Trump's repeated argument that he declassified all material he removed from the White House. It could also prove to be a key piece of evidence if prosecutors seek to show the former president was aware he should not be in possession of classified documents.