Planes sent Thursday to check the spot where Chinese satellite images showed possible debris from the missing Malaysian jetliner found nothing, Malaysia's civil aviation chief said, deflating the latest lead in the six-day hunt.

"There is nothing. We went there, there is nothing," Azharuddin Abdul Rahman told reporters in Kuala Lumpur.

Adding to the mystery, The Wall Street Journal reported that U.S. investigators suspect the plane flew on for four hours once it lost contact with air traffic controllers, based on data from the plane's engines that are automatically downloaded and transmitted to the ground as part of routine maintenance programs.

The report, based on two anonymous sources, raises questions as to why the Boeing 777 would have been flying without passive or active contact with the ground, and if anyone would have been in control during that time. U.S. counterterrorism officials are considering whether a pilot or someone else on board intentionally disabled the jetliner's transponders to avoid detection and divert it, the report said.

The hunt for the missing Malaysia Airlines flight 370 has been punctuated by false leads since it disappeared with 239 people aboard about an hour after leaving Kuala Lumpur for Beijing early Saturday.

The plane was heading northeast over the Gulf of Thailand toward Vietnam when vanished. The last message from the cockpit of the missing flight was routine: "All right, good night," was the signoff transmitted to air traffic controllers.

Shortly after that it fell off commercial radar, but military authorities say it might have turned west and flown into the northern stretches of the Strait of Malacca based on unconfirmed traces seen on its air defense radar.

Dozens of ships and aircraft from 12 nations have been searching the Gulf of Thailand and the strait, but no trace has been found. The search area has grown to 35,800 square miles (92,600 square kilometers), or about the size of Portugal.

If the Wall Street Journal report is confirmed, the search area will have to significantly expand.

The Chinese satellite imagery showing possible debris was not far from where the last confirmed position of the plane was between Malaysia and Vietnam. The images and coordinates were posted on the website of China's State Administration of Science, Technology and Industry for National Defense.

China's state Xinhua News Agency said the images from around 11 a.m. on Sunday appear to show "three suspected floating objects" of varying sizes in a 20-kilometer (12-mile) radius, the largest about 24-by-22 meters (79-by-72 feet) off the southern tip of Vietnam.

Li Jiaxiang, chief of the Civil Aviation Administration of China, said later China had yet to confirm any link between the suspected floating objects and the plane.

Pham Quy Tieu, deputy transport minister, told The Associated Press that the area had been "searched thoroughly" by forces from other countries over the past few days. Doan Huu Gia, chief of air search and rescue coordination center, said Malaysian and Singaporean aircraft were scheduled to visit the area again Thursday.

Malaysian authorities have come under fire for their handling of the search amid sometimes confusing and conflicting statements, including the time of the plane's disappearance. Officials had also said that five passengers had checked into the flight but did not board the plane, and their luggage had been removed, but later they said this was not true.

Malaysia's air force chief said Wednesday that an unidentified object appears on military radar records about 200 miles (320 kilometers) northwest of Penang, Malaysia, and experts are analyzing the data to determine whether the blip is the missing plane.

Authorities have not ruled out any possible cause, including mechanical failure, pilot error, sabotage and terrorism, and they are waiting to find any wreckage or debris to determine what went wrong.

Two U.S. Federal Aviation Administration technical experts and a regional representative are in Kuala Lumpur as part of a National Transportation Safety Board team supporting the investigation. Experts in air traffic control and radar are providing technical help, the board said.