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

That’s what the staff of the National Transportation Safety board found in its 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