Libyan authorities demanded an investigation on Thursday into whether human failings were to blame for thousands of deaths in the worst natural disaster in the country's modern history, as survivors searched for loved ones washed away by floods. A torrent unleashed by a powerful storm burst dams on Sunday night and hurtled down a seasonal riverbed that bisects the eastern city of Derna, washing multi-storey buildings into the sea with sleeping families inside. Derna Mayor Abdulmenam al-Ghaithi s