The pilots of two Alaskan sightseeing planes carrying passengers on a Princess Cruises’ shore excursion that collided in midair couldn’t see the other aircraft because airplane structures or a passenger blocked their views, says a report into the May 2019 tragedy.

Six people were killed in the crash, including Mountain Air Service pilot Randy Sullivan and his four passengers, and a passenger in a plane owned by Taquan Air, and 10 people suffered injuries when the aircraft converged at 1,021 meters.

An investigation by the US National Transportation Safety board, released this week, also said the pilots didn’t get electronic alerts about close aircraft because safety systems weren’t working properly.

The Ketchikan-based floatplanes carrying passengers from the same cruise ship, the Royal Princess, were returning from tours of Misty Fjords National Monument and collided just after noon over the west side of George Inlet.

The board was scheduled to meet Tuesday in Washington to make recommendations to prevent it from happening again.