PERTH, Australia: Search crews sent a robotic submarine deep into the Indian Ocean on Monday to begin scouring the seabed for the missing Malaysian airliner after failing for six days to detect any signals believed to be from its black boxes.

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

Crews have collected an oil sample and are sending it back to Australia for analysis. 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.

The unmanned underwater vehicle, the Bluefin 21, was launched from the Australian navy ship Ocean Shield, the US Navy said.