Thx - I had separate tickets, but the other passengers had thru tickets and we were *all* initially denied access to the T3 security since we were all told we were too late. Fortunately, AA intervened and let us thru. It was a common sense solution since there was absolutely no security line and we had I think 40-50 minutes until departure. We were all in J on both the AA and BA flights. However, I think there were other passengers on the flight who were behind us who were ultimately denied boarding since they were perhaps 5-10 minutes behind us still trying to get off.

I see how the airport/airline want to close boarding ahead of schedule so they can offload baggage, which I don't believe happens in the US. But I have always been under the impression - and it is an impression - that you just had to be at the boarding gate by the cutoff if I don't check luggage, i.e. there is a check in time for luggage and a boarding time for hand luggage only. On BA boarding passes it says that you must clear security by X, but this isn't on AA passes. So it would help passengers understand they have the potential to be denied boarding on AA if they don't arrive at security by a certain time.