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An oil slick spotted in the waters between Malaysia and Vietnam after the disappearance of Malaysia Airlines Flight 370 was determined not to be from an aircraft, the top investigator said Monday.

The investigator, looking into why two men boarded the flight with stolen passports, also said authorities had reviewed video footage from check-in to departure and found that “all security protocols have been complied with.”

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Almost three full days after the plane, a Boeing 777 with 239 people on board, disappeared over the South China Sea, there remained no confirmed wreckage and no explanation of what happened.

The video footage showed that the men were not of Asian appearance, said the investigator, Azharuddin Abdul Rahman, the head of the Malaysian Department of Civil Aviation. He also said that the search area is being expanded.

“We are looking at every possibility,” he told reporters in Kuala Lumpur, Malaysia.

The U.S. Navy said that a second destroyer, the USS Kidd, has joined the American search contingent. The USS Pinckney was already there. Both destroyers are equipped with two Seahawk helicopters and a patrol plane. China, Malaysia and Vietnam are among the other countries looking for clues.

Malaysian authorities were looking into the possibility of a stolen-passport syndicate, but it was not clear how that might be related to the flight.

Earlier on Monday, helicopters and ships raced toward a yellow object in the water that looked like a life raft, but it turned out to be the moss-covered cap of a cable reel and not from the plane.

— Erin McClam