Sonar vessels searching for Malaysia Airlines Flight MH370 in the southern Indian Ocean may have missed the jet, Australian Transport Safety Bureau’s Chief Commissioner Martin Dolan told News Online, an Australian website, Saturday. Dolan’s comments come just a day after authorities announced that a Chinese sonar-equipped vessel will join the search for Flight MH370 in late February.

Dolan reportedly said that crews are retracing their steps to be sure that they had not missed the plane, which is believed to be resting in the southern Indian Ocean floor. Dolan said that the difficult terrain of the ocean floor did not always allow the vessels to get a “good enough sonar image,” News Online reported.

“We’re taking another look because the areas where we haven’t been certain are large enough to contain an aircraft — which is why we’re going over them,” Dolan said, adding that “the sea floor is very rugged and complex.”

The hazards involved in the search operations were highlighted after search teams lost the sonar detector used in the operation. The deep water detector, or towfish, was lost after it hit an underwater volcano and sunk to the ocean floor last Sunday.

Dolan reportedly said that there was still an area “two-thirds the size of Tasmania” to search, but authorities were confident about finding the Boeing 777-200 before the operation ended.

“There was never a guarantee of success, but we still think there’s a high likelihood of success before we finish [in June],” Dolan reportedly said.

Flight MH370 disappeared on March 8, 2014, with 239 people on board during a flight from Kuala Lumpur to Beijing. So far, the first and only piece of physical evidence to be recovered from the plane is a flaperon that washed ashore on France's Réunion Island in the Indian Ocean — about 2,300 miles away from the current search area — in July 2015.

Dolan told News Online that ATSB was still awaiting the outcome of a French investigation into the position of the flaperon at the time of its separation from the plane.

“At this stage we’ve not heard anything from the French that would enable us to form a view about what position the flaperon was in when it separated from the aircraft which is the key question for us,” Dolan reportedly said. “It’s quite possible we won’t get anything definitive on that.”

Malaysia is expected to release another report on the second anniversary of the plane’s disappearance on March 8, 2016. However, Dolan said that ATSB is not aware of the contents of the report.