Investigators sifting through evidence from the missing Malaysia Airlines Flight 370 have said blackened debris recovered on Madagascar does not necessarily mean the plane crash was caused by an on-board fire.

The discovery of pieces of apparently burnt debris from the from the Boeing 777, which is thought to have plunged into the Indian Ocean with 239 people on board south-west of Australia on 8 March, 2014, had led to the theories the plane was brought down by a fire.

However, closer inspection of two fibreglass-honeycomb pieces showed the black discolouration had been caused by a reaction in resin that had not been caused by exposure to fire or heat, the Associated Press reported.

There had been some heat damage to the pieces but this appeared to be recent rather than caused in the crash more than two years ago. "There were three small areas of heat damage on one of the pieces which created a burnt odour. However, that odour suggested the heat damage was recent," the Australian Transport Safety Bureau (ATSB) said in a statement.

"It was considered that burning odours would generally dissipate after an extended period of environmental exposure, including saltwater immersion, as expected for items originating from" the missing plane, the statement added.

Evidence discovered earlier this month has shown flight MH370 plunged into the ocean in a "death dive". Analysis of the Boeing 777 wing flap, washed ashore in Tanzania, revealed it was not deployed at point of impact, eliminating suggestions the aircraft with 239 passengers on board could have made a controlled landing.

It confirms previous analysis of the automated flight signals that the doomed aircraft could have dropped from a height of 35,000ft at a speed of up to 20,000ft a minute before crashing into the sea.

The ATSB has led the A$180m (£103.8m, $134.9m) search, over 120,000km sq of sea floor, since the aircraft disappeared on 8 March 2014.

A sonar search of 120,000 square kilometers (46,000 square miles) of seabed which is calculated to be the most likely crash site in the southern Indian Ocean is almost complete without any trace of the plane being found.