SEARCH crews have sent a robotic submarine deep into the Indian Ocean for the first time to begin scouring the seabed for the missing Malaysian airliner after no signals from its black boxes were detected for six days.

Meanwhile, officials are investigating an oil slick about 5,500 meters from the area where the last underwater sounds were detected, said Angus Houston, head of the joint agency that is coordinating the search off Australia’s west coast.

Crews collected an oil sample and are sending it back to Australia for analysis, a process that will take several days. Houston said it does not appear to be from any of the ships in the area, but cautioned against jumping to conclusions about its source.

Bluefin 21 was launched from the Australian navy ship Ocean Shield.

The move comes after a series of underwater sounds over the past two weeks consistent with signals from an aircraft’s black boxes.

The devices emit “pings” so they can be more easily found, but their batteries only last about a month and are now believed dead.

“Today is day 38 of the search,” Houston told a news conference. “We haven’t had a single detection in six days, so I guess it’s time to go under water.”

Australian Prime Minister Tony Abbott raised hopes last week when he said authorities were “very confident” the four strong underwater signals that were detected were from the black boxes of the Malaysia Airlines plane, which disappeared on March 8 during a flight from Kuala Lumpur, Malaysia, to Beijing with 239 people on board, mostly Chinese.

But Houston warned that while the signals are a promising lead, the public needs to be realistic about the challenges facing search crews in the extremely remote, deep patch of ocean — an area he called “new to man.”

“I would caution you against raising hopes that the deployment of the autonomous underwater vehicle will result in the detection of the aircraft wreckage. It may not,” Houston said. “However, this is the best lead we have, and it must be pursued vigorously.”

The Ocean Shield had been dragging a US Navy device called a towed pinger locator through the water to listen for any sounds from the black boxes’ beacons.

The Bluefin sub takes six times longer to cover the same area as the ping locator, and the two devices can’t be used at the same time. Crews had been hoping to detect additional signals before sending down the sub, so they could triangulate the source and zero in on where the black boxes may be.

The submarine will take 24 hours to complete each mission: two hours to dive to the bottom, 16 hours to search the ocean floor, two hours to return to the surface, and four hours to download the data, Houston said.

The black boxes could contain the key to unraveling the mystery. Investigators believe the plane went down in the southern Indian Ocean based on a flight path calculated from its contacts with a satellite and an analysis of its speed and fuel capacity. But they still don’t know why.

A visual search for debris on the ocean surface continued yesterday over 47,600 square kilometers of water about 2,200 kilometers northwest of Perth. A total of 12 planes and 15 ships joined the search.