Using the text, answer  shortly what happened on September 11, 2001?

Context: The September 11 attacks, commonly known as 9/11, were four coordinated suicide terrorist attacks carried out by the militant Islamist extremist network al-Qaeda against the United States on September 11, 2001. That morning, nineteen terrorists hijacked four commercial airliners scheduled to travel from the East Coast to California. The hijackers crashed the first two planes into the Twin Towers of the World Trade Center in New York City and the third into the Pentagon in Arlington County, Virginia, near Washington, D.C. The fourth plane was similarly intended to hit a federal government building in D.C., but crashed in a field following a passenger revolt. The attacks killed nearly 3,000 people and instigated the global war on terror.

Answer: The September 11 attacks, commonly known as 9/11, were four coordinated suicide terrorist attacks carried out by the militant Islamist extremist network al-Qaeda against the United States on September 11, 2001 . The attacks killed nearly 3,000 people and instigated the global war on terror . It remains the deadliest terrorist attack in human history .