To this day, just 59% of the victims have been identified, mostly through DNA. No trace has been found of the remaining 41 percent, or 1,115 people, according to the medical examiner's office.
Over the years, authorities have recovered 21,906 human remains of which 63 percent have been identified.
The 9/11 attack killed 2,753 people in New York, including 403 police and firefighters.
At the Pentagon, where American Airlines Flight 77 crashed at 9:37 a.m. on September 11, President Obama will speak at a private observance for family members of the 184 people who died there.
In southwestern Pennsylvania, it is only expected to take 18 minutes to lay a wreath and read the names of 40 people, beginning at 9:45 a.m. and ending at 10:03 a.m. That is the time United Airlines Flight 93 crashed into a field near Shanksville.
A bomb sniffing dog named Sirius also died at the WTC.
In 2007, the New York City medical examiner's office began to add people to the official death toll who died of illnesses caused by exposure to dust from the site. The first such victim was a woman who had died in February 2002 from a lung condition. In 2009, a man who died in 2008 was added, and in 2011 a man who died in 2010.
Many more people have died since as a result of exposure to 9/11 dust at Ground Zero.
Recordings of 9-1-1 calls show how New York City's emergency operators and dispatchers did their jobs on 9/11.
http://channel.nationalgeographic.com/channel/videos/how-it-was-voices-of-911
