You can buy tickets at the door, but the line can back up pretty extensively. Paying in advance allows you to walk directly to the entrance for security screening.
You can get on line to buy tickets at the door or at one of the kiosks on the side of the building when they are functioning, or at 911memorial.org. Various 3rd party passes will still require you to redeem actual tickets at the ticket counter. After getting tickets or if you bought and printed them from the website you are still required to get on another line for security screening.
Even if you've bought tickets in advance, it does not make sure that you can just step in. You still have to wait in line. We had city pass tickets and had to wait even longer than those visitors who had no ticket and needed to buy some...
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