Investigators examining the debris from EgyptAir 990 are still working on the theory that the aircraft was deliberately crashed into the Atlantic Ocean, according to reports from the United States.

Officials at the National Transportation Safety Board (NTSB) say investigations have yielded no evidence that alternative explanations, such as an explosion or mechanical failure, caused the aircraft to crash off the coast of Massachusetts on 31 October, killing all 217 people on board.

NTSB spokesman Keith Holloway said: "We have not seen any signs of mechanical failure," although he confirmed "that does not rule it out completely".

The NTSB has said on a number of occasions that no evidence has been uncovered to refute the hypothesis that the plane was deliberately brought down by a member of the crew.

Investigations have focused on a co-pilot Gamal el-Batouty, sparking angry reactions from the Egyptian governments, the country's pilots association and Mr el-Batouty's family.

Reconstructing the aircraft

EgyptAir dismissed reports that the crash was connected with the suicide of the co-pilot, citing problems with the aircraft's tail apparatus.

The NTSB has also confirmed that there are no plans to reconstruct the aircraft - almost 70% of which has been retrieved from the ocean floor.

This, aviation experts say, highlights the fact that investigators are not looking for alternative explanations to replace the theory that the crash was deliberate.

Engines shut off

"In think they're fairly convinced they know what happened based on the radar data, flight data and voice data," an aviation safety consultant and former TWA pilot, Bary Shiff, told Associated Press news agency.

Previous information from the plane's recorders has shown that there was no mechanical failure or explosion and that the fatal dive appears to have been deliberately initiated

A former NTSB supervisor, Jim Danaher, concurred.

"You can take all sorts of oddball hypotheses to make the scenario different than the one that is pretty obvious," Mr Danaher, who retired from the NTSB in 1998 after 28 years.

"I think that is contributing to the board's lack of need to gather up and assemble every piece of the thing - the pieces won't tell you anything to counter that theory."

Last month, a Navy submarine mapping wreckage from the crash located the plane's remaining engine on the sea bed.

According to the NTSB, the view of the submerged wreckage showed damage indicating the engine was generating little or no power when the aircraft hit the water, supporting evidence from the flight data recorder that the engines were shut off during the crash.

In November, the NTSB had sought to turn the investigation over to the FBI, required when evidence points to a criminal act. Protests from Egypt forced an indefinite postponement of that decision.

But there are reports that the FBI has continued to quietly assist safety investigators.