geneva veteran prosecutor carla del ponte sign unite nations syria investigation monday criticize u.n. security council tell syria ambassador government use chemical weapons former swiss attorney general go prosecute war crimes rwanda former yugoslavia say august resign u.n. commission inquiry syria lack political back bid farewell u.n. human right council set commission inquiry six years ago del ponte say quit frustration could obtain international community security council resolution put place tribunal ad hoc tribunal crimes commit syria say seven years crime syria total impunity acceptable del ponte tell swiss newspaper last month enough evidence exist convict president bashar al-assad war crimes departure leave two remain commissioners inquiry karen koning abuzayd chairman paulo pinheiro say eventually great many people would answer act sooner stop carnage deadlock security council syria reprehensible time bewilder tell human right council leave council del ponte tell syria ambassador right quickly reach conclusion assad government use chemical weapons attack town khan sheikhoun april mister ambassador say say opinion base elements already syrian government responsible today confirmation official commission inquiry ask justice ask justice victims syria investigator del ponte sign sting