How did the 9/11 hijackers get into the cockpit?

There are surveillance camera photos of Atta and another hijacker as they waited to proceed through the screening area before boarding that flight. Neither of them were disguised as airline flight crew.

If I'm not mistaken, there was no requirement prior to 9/11 for aircraft to have entry proof doors or locks. Most did have a door and a very flimsy slip bolt lock to allow the crew a bit less noise and bother. The typical doors were 1/8" fiberboard with thin, flat metal stiffeners and edge framing. The slip bolt lock was similar to a sliding bolt lock used for residential gates. Very flimsy.

The standard fiberboard flightdeck doors of that time could easily be broken into by slamming your shoulder into it once or twice. Most of the time before 9/11 those doors were closed but not locked on almost all flights. That was the norm. There was no reason to believe anything was going to happen.

After the 9/11 commission's investigation, security doors with tamper proof cylinder locks were required and were installed.

Glenn