A Malaysian official announced on Monday that the miles-long oil slick spotted on Saturday did not come from missing Malaysian Airlines Flight 370, which vanished off the coast of Vietnam early Saturday morning with 239 people on board.

According to the AFP and the BBC, the oil slicks were tested in a lab and did not match samples of airplane fuel.

At least 40 ships and 34 airplanes from nine different countries have joined in the search for missing plane, which Malaysia's civil aviation chief Azharuddin Abdul Rahman described on Monday as an "aviation mystery."

"Unfortunately we have not found anything that appears to be objects from the aircraft, let alone the aircraft," he said at a news conference on Monday. "As far as we are concerned, we have to find the aircraft, we have to find a piece of the aircraft if possible."

Floating debris found on Monday, initially believed to be a life raft from the plane, turned out to be an unrelated object.

A source from the investigation in Malaysia told Reuters that the plane likely broke apart mid-air, probably because of mechanical failure.

"The fact that we are unable to find any debris so far appears to indicate that the aircraft is likely to have disintegrated at around 35,000 feet," the source said.

There is some hope that the plane's flight recorder will be found floating in the ocean, according to Commander William Marks of the U.S.'s Seventh Fleet, which has joined the search. From the BBC:

"In calm seas, if there were a soccer ball [football] or a basketball floating in the water, the radar could pick it up. They [flight recorders] typically have a radio beacon and so for example our P3 [radar] - if they are flying within a certain range of that - will pick up that radio beacon. We have not yet picked up anything, but that's typically what those black boxes contain."

Meanwhile, Malaysian police have identified one of the two people who boarded the flight using a stolen passport. The passenger is reportedly not from Malaysia, but no other information about his identity has been released.

[Image via Getty]