In all, the attacks by al-Qaida militants killed 2,977 people, including many financial workers at the World Trade Center and firefighters and police officers who had rushed to the burning buildings trying to save lives.

The attacks reverberated globally and altered the course of U.S. policy, both domestically and overseas. It led to the " Global War on Terrorism " and the U.S.-led invasions of Afghanistan and Iraq and related conflicts that killed hundreds of thousands of troops and civilians.

While the hijackers died in the attacks, the U.S. government has struggled to conclude its long-running legal case against the man accused of masterminding the plot, Khalid Sheikh Mohammed. The former al-Qaida leader was arrested in Pakistan in 2003 and later taken to a U.S. military base at Guantnamo Bay, Cuba, but has never received a trial.