World Health Organization left quarantine in Wuhan on Thursday to begin a heavily scrutinised probe into the origins of the Covid-19 pandemic, after Washington urged a "robust and clear" investigation.The group started a two-week quarantine on arrival on January 14 in the central Chinese city where the first known cluster of virus cases emerged in late 2019.Wearing masks, they peered at the ranks of waiting media from the window of a bus which whisked them from the quarantine to another hotel on Thursday -- although it was not immediately clear when and where their investigation will start."So proud to graduate from our 14 days...