As the hulking passenger jet approached London's busiest airport after a long flight from China, the inexplicable happened: Both of the plane's engines sputtered and essentially died.

There was little the British Airways pilots could do to keep the Boeing 777 in the air. Within seconds, the twin-engine jet pancaked spectacularly to the ground. The plane was wrecked beyond repair.

Although no one was killed in the Jan. 17 crash, investigators are facing extreme pressure to determine what brought down the 777 in an accident that has quickly become one of aviation's modern mysteries.

The wide-body 777 is one of the world's most popular long-haul jets, ferrying tens of thousands of passengers a day across the globe. The crash has raised questions about how airlines operate an increasing number of long flights over remote and harsh areas of the world.

British authorities have not said much publicly about the accident. They released a report Monday saying they suspected that the plane's fuel flow became restricted somewhere between the engines and the fuel tanks. They did not indicate what they thought caused the blockage. Ice collecting in or near an engine component has emerged as the prime suspect, according to sources familiar with the investigation who spoke on condition of anonymity.

It is ironic, outside experts said, that investigators have not figured out what caused the jet to crash about 1,000 feet short of a runway at Heathrow Airport. Investigators expected a quick resolution because they have obtained more data and information about the accident than any they can recall in aviation history, according to sources and outside experts.

"This is a great mystery, and I never expected this accident to be this difficult to solve, given the state-of-art tools on the plane and the fact that the aircraft was largely intact," said Bill Voss, president of the Flight Safety Foundation, an organization that advocates for improving aviation safety. "This has potentially broad implications that go beyond this one airplane, depending on what they find."

A spokesman for Rolls-Royce, which made the plane's engines, declined to comment.

Spokesmen for Boeing and British Airways also declined to comment, citing company policies against making public statements before investigators have concluded their work.

British Airways Flight 38 began like any other for its 16 crew members and 136 passengers when it took off from Beijing about 10 a.m. Jan. 17.

The plane headed over Siberia and skirted the Arctic, far from emergency airports. For years, regulators have generally limited such flights on twin-engine planes because they worried about engine failure. But engines have become so reliable that U.S. regulators have steadily loosened those limitations, including restrictions on flights over the North and South poles. The Boeing 777, especially versions designed to fly extremely long distances, has become the workhorse of many of those routes.

During portions of the British Airways flight, the outside temperature was as low as minus-49 degrees. Experts said such temperatures should not have been a particular concern to the crew.

The lowest recorded temperature of the plane's fuel was minus-29 degrees, well above the freezing point of minus-70. Investigators found that the fuel did not have "excessive" amounts of water in it, which can cause engines to freeze and block fuel lines.

Toward the end of an otherwise uneventful 10 1/2-hour flight, the plane was nearing Heathrow about 10:40 a.m. London time. About 800 feet above the ground, the plane was configured to land, and the copilot had taken the controls.

About this time, the plane's auto-throttle system commanded the engines to increase thrust to keep the plane on its proper glide path to the runway.

The engines initially responded, but then the right engine's thrust subsided. A few seconds later, thrust on the left engine dropped off. The engines did not quit, but their thrust was too weak to keep the plane airborne. The plane slowed and lost lift. Then it smacked to the ground, according to investigators and witnesses.

At first, investigators thought the plane might have experienced a computer glitch that led to the reduction in thrust.

But the recorders quickly disproved that theory, revealing no errors in the computer signals sent from the cockpit to the engines, according to reports and sources.

When they took apart the engines, investigators found evidence on pumps that the engines had been starved for fuel in the moments before the crash. That led them to conclude that the fuel supply had become blocked somewhere between the plane's tanks and its engines.

Sources familiar with the probe said engineers suspect that ice collected in or near a fuel-oil heat exchanger on each engine, blocking the fuel supply. The heat exchanger uses cold fuel to cool hot engine oil, and the hot engine oil to warm fuel before it is injected into the engines. The heat exchanger is the only point in the system that engineers have not yet eliminated as the potential bottleneck, the sources said.

But the 777's fuel contained additives designed to prevent it from freezing. Also, each engine should have been drawing fuel from a separate tank. The chances of ice breaking off in separate fuel tanks and blocking the fuel supply in each engine at nearly the same moment is almost too small to comprehend, said John Goglia, a former member at the National Transportation Safety Board, which investigates air crashes in the United States.

"This isn't supposed to happen," Goglia said. "These are two independent systems."