But the international search effort remains full of “hope and optimism,” said Capt. Jamie Frederick of the U.S. Coast Guard, adding that officials are still planning to recover a live crew.

“This is a search and rescue mission, one hundred percent,” Frederick said Wednesday afternoon. “We’re right in the middle of a search and rescue case, so I don’t want to get into a discussion about when that would end.”

Several Canadian P-3 aircraft detected underwater noises in the area where crews are looking for the Titan submersible, and operations were redirected there, the U.S. Coast Guard’s First District reported early Wednesday. Those efforts “have yielded negative results but continue,” the Coast Guard tweeted.

Carl Hartsfield, a senior manager at the Woods Hole Oceanographic Institution, said at a news conference that officials were conducting an acoustic analysis of what was “described as banging noises.” The sounds could have originated from other constructed sources or animals.

“We don’t know what they [the noises] are, to be frank with you,” Frederick said.

The search and rescue efforts remain focused on the area where the noises were heard Tuesday and Wednesday.