WASHINGTON — The pilots of two Alaskan sightseeing planes that collided in midair couldn’t see the other aircraft because airplane structures or a passenger blocked their views, and they didn’t get electronic alerts about close aircraft because safety systems weren’t working properly.

That’s what the staff of the National Transportation Safety board found in their investigation of the May 2019 crash, which killed six people.

The board is meeting Tuesday in Washington to determine a probable cause of the crash and make recommendations to prevent it from happening again.

Mountain Air Service pilot Randy Sullivan and his four passengers, and a passenger in a plane owned by Taquan Air were killed. Ten people suffered injuries when the aircraft converged at 3,350 feet (1,021 meters).

The Ketchikan-based floatplanes carrying passengers from the same cruise ship, the Royal Princess, were returning from tours of Misty Fjords National Monument.

Mountain Air’s single-engine de Havilland DHC-2 MK 1 Beaver and Taquan’s larger turboprop de Havilland DHC-3 Otter collided just after noon over the west side of George Inlet.