PERTH, Australia (AP) — A robotic submarine completed its first successful scan of the seabed Thursday in the hunt for the missing Malaysian plane, and investigators were analyzing the sub's data while also trying to identify the origins of a nearby oil slick. The Bluefin 21's first two missions were cut short by technical problems and deep water, but the unmanned sub finally managed to complete a full 16-hour scan of the silt-covered seabed far off Australia's west coast, the search coordination center said on Thursday. [...] officials in the western city of Perth were analyzing an oil sample that search crews collected earlier this week about 5.5 kilometers (3.4 miles) from an area where equipment picked up underwater sounds consistent with an aircraft black box. In addition to finding the plane itself, investigators want to recover the black boxes in hopes the cockpit voice and flight data recorders can explain why the plane lost communications and flew so far off-course before disappearing.