There are two memorial events being held that I know of.
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Sept. 11, 5:46 AM - 7:28 AM SL time, World Trade Center
The World Trade Center build is very impressive, featuring the Wall of Names. You can leave flowers and donations, and they will have an event tomorrow from the time the first plane hit until the towers fell. They have had 1000 visitors in the past 36 hours.
http://slurl.com/secondlife/World%20Trade%20Center/172/82/26
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Sept. 10th, starting 11:40 PM, Melissa's Blue Angel Memorial Community Garden
Please come and join us in remembering our fallen officers, firefighters and first responders starting Monday, September 10th at 11:40 p.m. SL time. Visitors from all over the world will be paying their respects in various languages at the Melissa's Blue Angel Police and Fire Memorial Community Garden.
http://slurl.com/secondlife/Pacidlux/68/236/31