On September 11, 2001, on America's single-worst day of terrorism, nearly 3,000 people were killed as 19 al-Qaida members hijacked four passenger jetliners, sending two of the planes smashing into New York's World Trade Center, one into the Pentagon and the fourth into a field in western Pennsylvania.

Five years ago: On his first 9/11 anniversary as president, Barack Obama urged Americans to come together in service just as they united after the terrorist attacks.

Ten years ago: Parents and grandparents of those lost on September 11 stood at the World Trade Center site and marked the third anniversary of the attacks.