PERTH, Australia (AP) -- A robotic submarine headed back down into the depths of the Indian Ocean on Friday to scour the seafloor for any trace of the missing Malaysian jet one month after the search began off Australia's west coast, as data from the sub's previous missions turned up no evidence of the plane.
It was the fifth attempt by the Bluefin-21 unmanned sub to find wreckage or the black boxes from Flight 370 in a distant patch of seabed. The sub, which can create sonar maps of the ocean bottom, has now covered 110 square kilometers (42 square miles) of the silt-covered seabed, but has thus far found nothing, the search coordination center said. The sub's last mission hit a record depth beyond its recommended diving parameters, which can potentially cause risk to the equipment, the U.S. Seventh Fleet said in a statement. However, it is being closely monitored.
Officials are desperate to find some physical evidence that they are searching in the right spot for the Boeing 777, which vanished March 8 with 239 on board on a flight from Kuala Lumpur, Malaysia, to Beijing. A weeks-long search of the ocean surface hasn't turned up a single piece of debris, and officials on Thursday determined that an oil slick found in the search zone did not come from the plane.
The Bluefin is searching a remote stretch of ocean floor about 4,500 meters (15,000 feet) deep in an area where sound-locating equipment picked up a series of underwater sounds consistent with an airplane's black box, but it went down to 4,695 meters (15,404 feet) during mission four. Prime Minister Tony Abbott has said officials are “very confident” the sounds came from the Malaysian jet's cockpit voice and flight data recorders, but finding the devices in such deep water is an incredibly difficult task.
Radar and satellite data show the plane flew far off-course and would have run out of fuel in a remote section of the Indian Ocean. Planes and ships have been scouring the ocean surface for a month, to no avail.
On Friday, 11 planes and 12 ships were continuing the surface search across about 52,000 square kilometers (20,000 square miles) of ocean. The U.S. alone has flown 35 missions, racking up 319 hours of flight time over nearly 450,000 nautical miles of ocean, according to the Seventh Fleet.
Angus Houston, who is heading up the search effort, said earlier this week that the hunt for floating debris would be ending within days, because it is unlikely that anything will be found. But the search coordination center said the effort would continue into next week, more than six weeks after the plane vanished.
Malaysia's defense minister, Hishamuddin Hussein, confirmed that the search would continue through the Easter weekend, but acknowledged that officials would have to rethink their strategy at some point if nothing is found.
“There will come a time when we need to regroup and reconsider, but in any event, the search will always continue. It's just a matter of approach,” he said at a news conference in Kuala Lumpur on Thursday.
The U.S. Navy's unmanned sub cut short its first mission on Monday because it exceeded its maximum operating depth of 4,500 meters (15,000 feet). Searchers moved it away from the deepest waters before redeploying the sub to scan the seabed with sonar to map a potential debris field.
But the search coordination center said Thursday that officials are now confident the sub can safely go deeper than was thought, allowing it to cover the entire search area, which has been narrowed based on further analysis of the four underwater signals previously detected.
Associated Press writers Kristen Gelineau in Sydney and Rod McGuirk in Canberra, Australia, and Eileen Ng in Kuala Lumpur, Malaysia, contributed to this report.
Follow Margie Mason on Twitter at twitter.com/MargieMasonAP