Poland has declared a week of national mourning following the death of its president and many of its political and military leaders in a plane crash in Russia.<br /><br />In one of the country's worst days since the second world war, 97 people were killed when their plane went down in heavy fog.<br /><br />The group had been travelling to western Russia for a memorial service marking the 70th anniversary of the Katyn massacre, in which Russian forces killed more than 20,000 Polish prisoners of war.<br /><br />Church leaders and some of the families of those who had died in the massacre were also on board.<br /><br />Vladimir Putin, the Russian prime minister has promised a swift investigation into the incident and it leading the probe himself.<br /><br />Al Jazeera's Jonah Hull reports from Warsaw. (Apr 11, 2010)
