Teams from the US space agency NASA have gone to California and Arizona after reports of debris from the Columbia disaster landing there. If confirmed, the reports would suggest that the space shuttle began to break up earlier than previously thought. On Tuesday, President George W Bush paid tribute to each of the seven astronauts killed in Saturday's disaster and vowed that the "cause of discovery" would go on. The remains of one, Israeli Ilan Ramon, have now been positively identified and are due to be returned to Israel for burial. The Columbia was dramatically filmed disintegrating in the skies over Texas where much of the 3,500 debris sites are located.