A helicopter pilot may not have heard a radio call from another helicopter shortly before a deadly mid-air collision in Australia, a report has suggested.
The interim accident report also said the pilots may not have been able to see each other.
A British couple were among four people killed when the two helicopters collided off the Gold Coast in January.
One helicopter crashed, while the other managed to land with heavy damage.
The helicopters were being used for short sightseeing trips by Sea World Helicopters. One was coming in to land, while the other had just taken off.
“The report details the calls made by the pilot of the returning helicopter and as they tracked south… that they saw passengers boarding the second helicopter as it was preparing to depart,” Australian Transport Safety Bureau (ATSB) Chief Commissioner Angus Mitchell said.
“The pilot of the returning helicopter recalled that the assessment was that the departing helicopter would pass behind them and that they did not recall the pilot making a standard taxiing call, thereby announcing their intentions to depart.”
Mr Mitchell said this did not mean a taxiing call was not made, and that the ATSB would carry out a detailed analysis of radio activity at the time of the crash.
Footage later emerged of one passenger trying to warn the pilot that another helicopter was approaching.
But the ATSB said this did not mean that the other helicopter was visible to either pilot.
British victims, Ron and Diane Hughes from Cheshire, were holidaying in Queensland at the time, police said.
An Australian woman, Vanessa Tadros, 36, also died in the crash, and her 10-year-old son was among the survivors but seriously injured.
The fourth fatality was Ashley Jenkinson, 40, an experienced pilot who lived in the area, but was reportedly originally from England.
