NEW : U.S. Navy commander : Initial optimism over pulse signals becomes more cautious . Search officials call new pulse signals the best indication so far they are on the right track . Comment: Finding an answer will take time , officials warn .
NEW : U.S. Navy commander : Initial optimism over pulse signals becomes more cautious . Search officials call new pulse signals the best indication so far they are on the right track . Without wreckage from the missing plane , nothing is certain for now . Finding an answer will take time , officials warn .