Mark Coles profiles the muckraker's muckraker, American journalist Seymour Hersh, who has challenged the official story of how Osama Bin Laden was killed.

The Pulitzer Prize-winning writer is one of the world's most famous investigative reporters. He exposed both the My Lai massacre, when US soldiers killed hundreds of civilians during the Vietnam war, and the abuse of Iraqi prisoners by US soldiers at the Abu Ghraib prison.

To some he is a hero - a tenacious investigator of uncomfortable truths. To others he is a fantasist - a gullible journalist who sees conspiracy and cover-up wherever he looks.

At 78 he shows no sign of slowing down. Now Mark Coles investigates the investigator.

Producers: Hannah Barnes and Joe Kent.