The Australian transport bureau (ATSB) interim report into a midair collision that killed four people found that the pilot of a helicopter did not hear a radio call from the other pilot that he was lifting off and had visibility problems that caused him not to see the helicopter as it flew toward him, The Guardian reported.

Britons Diane Hughes and Ron Hughes, along with a 36-year old woman from New South Wales and one of the pilots, died in the crash between the two sightseeing helicopters on January 2. The accident occurred over Main Beach, near Sea World, in suburban Gold Coast, Queensland.

The report noted that at 1:155 p.m., the pilot of a helicopter referred to as XKQ began the sightseeing tour and was flying over the water approaching a sandbar near the helipad. The other helicopter piloted by XH9, who survived the crash, said that they did not hear a taxiing call over the radio from XKQ stating that it was lifting off.

“This does not necessarily mean that a taxi call was not made and this topic will be subject to detailed analysis by the ATSB investigation. The pilot of XH9 also reported that they did not see XKQ depart from the park helipad,” the report said.

The report pointed out that the two passengers on XH9 were the ones who first saw XKQ approaching. “Understanding the helicopter would only get closer, at least one passenger attempted verbal guidance to the pilot.”

The report added: “As the verbal guidance did not work, and anticipating a potential collision, one passenger physically alerted the pilot… The pilot of XH9 later recalled being alerted to the other helicopter by a passenger, but the pilot did not see XKQ approaching prior to the collision.”

At 23 seconds into XKQ’s flight, at approximately 130 feet above the ground, the two helicopters collided head-on, resulting in the deaths of three passengers and one pilot.

“The ATSB has released this preliminary report to detail the circumstances of this tragic accident as we currently understand them but it is important to stress that we are yet to make findings,” ATSB commissioner Angus Mitchell said, according to the news outlet. “Our findings as to the contributing factors to this accident, and the analysis to support those findings, will be detailed in a final report to be released at the conclusion of our investigation.”