‘We still don't know what happened to flight’

The fate of the missing Malaysian Airlines jet that disappeared three days ago with 239 people aboard remained a mystery Monday as the latest possible clues to the plane's whereabouts were discounted, authorities said.

Tests on two oil slicks off the coast of Vietnam revealed no connection to the flight, investigators said. One piece of floating, yellow debris turned out to be moss-covered trash; another piece seen from the air Sunday night could not be located Monday.

Malaysia's civil aviation head, Azharuddin Abdul Rahman, speaking to reporters in Kuala Lumpur Monday, said that investigators have so far not found anything that could be part of the missing plane.

Vietnam sent helicopters to investigate the floating "yellow object" that had been suspected of being a life raft from flight MH370 bound for Beijing. Separately, six planes and seven ships from Vietnam have so far been unable to find an object spotted by a low-flying plane on Sunday afternoon, said Doan Huu Gia, the chief of Vietnam's search and rescue coordination center.

Vietnamese officials had said they believe the object is one of the plane's doors, according to local news media reports.

Vietnam civil administration chief Pham Viet Dung said search teams from several countries sent boats to the area about 56 miles south of Tho Chu island, in an area where an oil slick was spotted Saturday. Malaysian maritime officials found some oil slicks in the South China Sea and sent a sample to a lab to see if the oil came from the plane, but it did not.

Flight MH370 vanished early Saturday two hours into a scheduled six-hour flight from Kuala Lumpur. A Malaysian official said Sunday the plane may have tried to return before disappearing.

Air force chief Rodzali Daud said military radar indicated the flight "may have made a turn back," but he did not say how far it got. "We are trying to make sense of this," Daud said.

Dozens of ships and aircraft have failed to find any piece of the missing Boeing 777 as investigators pursue "every angle" to explain its disappearance, including hijacking, Malaysia's civil aviation chief said Monday.

The U.S. Navy has provided the USS Pinckney, a guided-missile destroyer that carries two MH-60R helicopters, and a P-3C Orion with long-range search, radar and communications capabilities.

Reuters, citing what it called a senior source involved in the investigation, said the probe is focusing on the possibility that the plane disintegrated in the air.