By Julie Makinen, Los Angeles Times

BEIJING — Australian authorities coordinating the hunt for Malaysia Airlines Flight 370 said Thursday that lab tests have shown an oil slick found in the search area last Sunday is not aircraft engine fuel or hydraulic fluid.

Searchers are now using a robotic submarine, the Bluefin-21, to look for wreckage on the floor of the Indian Ocean more than 1,000 miles northwest of Perth, Australia.

The Bluefin-21 has a rated depth of 4,500 meters, or 2.8 miles, and shortly after it was deployed on its first run this week, it returned to the surface several hours later because it reached that depth and an automatic safety system sent it back up.

However, search coordinators said Thursday that the submarine’s owner-operator, Phoenix International, which has a contract with the U.S. Navy, has determined that the unmanned vehicle can operate at deeper depths.

“This expansion of the operating parameters allows the Bluefin-21 to search the sea floor within the predicted limits of the current search area,” authorities said.

David Kelly, CEO of Bluefin Robotics, said Monday that Phoenix had upgraded the Bluefin-21 just prior to its deployment to the Indian Ocean and had tested it in Hawaii.

Authorities said previous U.S. Navy estimates that it would take six weeks to two months for the Bluefin-21 to complete its survey of the designated search area were incorrect, but they did not give a revised time frame.

“Since the U.S. Navy provided comment some days ago, the underwater search has been significantly narrowed through detailed acoustic analysis conducted on the four signal detections” made by the towed pinger locator on the Australian ship Ocean Shield, authorities said in a statement. “This analysis has allowed the definition of a reduced and more focused underwater search area.”

Flight 370 vanished March 8 en route from Kuala Lumpur to Beijing with 239 people aboard.

AFP Photo/Chaideer Mahyuddin