Visitors are greeted by military members at the Pentagon Observance Ceremony on Sept. 11. Visitors are greeted by military members at the Pentagon Observance Ceremony on Sept. 11. More than 13,000 people attended the service to remember those who lost their lives one year ago when terrorists crashed a commercial airliner into the Department of Defense Headquarters. ( SHARE: Download: Full Size (0.03 MB) Credit: VIRIN: 117342-D-ELW01-092.jpg Photo Gallery