NEW : U.S. Navy commander : Initial optimism over pulse signals becomes more cautious . On Monday , U.S. Navy Commander William Marks told CNN from the search operation that the inability of the Ocean Shield 's pinger locator to find the signals again more than a day since the last reception caused initial optimism to become `` more and 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 . Comment: Finding an answer will take time , officials warn .