It has been 15 years since the September 11 attacks, when four passenger planes, operated by two major US air carriers were hijacked by 19 members of al-Quaeda, killing 2,996 people and injuring more than 6,000.

An explosion is seen the south tower of the World Trade Center as the hijacked United Airlines Flight 175 from Boston crashes into the building Spencer Platt/ Getty Images

Two of the planes, American Airlines, Flight 11 and United Airlines, Flight 175, were deliberately crashed into the upper floors of the North and South towers of the World Trade Center (WTC). Flight 11 hit floors 93 to 99 of the North Tower (1 WTC) at 8.46 m local time and Flight 175 struck floors 77 to 85 of the South Tower (2 WTC) 17 minutes later at 9.03 am. In under two hours, both 110-storey towers had collapsed. Between 16,400 and 18,000 people were in the WTC complex when the towers were struck. Of those, the vast majority evacuated safely, but many did not.

A third plane crashed into the Pentagon, the headquarters of the United States Department of Defense in Arlington, Virginia, leading to a partial collapse of the buildings western side. After learning about the other attacks, passengers on the fourth hijacked plane, Flight 93, intercepted the hijackers and the plane was crashed into an empty field in western Pennsylvania about 20 minutes by air from Washington, DC.

The 9/11 attacks are a powerful memory for Americans, and for the world. According to poll conducted by Pew Research Center, 91% of the adults that were surveyed remember exactly where they were or what they were doing when they heard news. Thousands of volunteers went to Ground Zero to help with the rescue, recovery and clean-up efforts. "We will rebuild. We're going to come out of this stronger than before, politically stronger, economically stronger. The skyline will be made whole again." Rudy Giuliani, mayor of New York City on the day of the fatal events.

US President George W Bush was informed of the attacks by House Chief of Staff Andrew Card while conducting a reading seminar at the Emma E Booker Elementary School in Florida. Bush continued the reading lesson, before returning to Washington. Writing in his book, Decision Points, Bush describes the contrast between what was happening in New York City and the innocent children sitting in front of him. "I looked at the faces of the children in front of me. I thought about the contrast between the brutality of the attacks and the innocence of those children. Millions like them would soon be counting on me to protect them. I was determined not to let them down." Bush returned to Washington after the reading, where he was joined by his aides.

Tons of wreckage – twisted steel beams weighing up to 40,000 pounds, chunks of concrete smelling of smoke, a crushed fire engine, a dust-covered airline slipper – were salvaged from the World Trade Center site for preservation in the weeks after the attacks. The New York City Fire Department deployed 200 units (half of the department) to the World Trade Center. The New York City Police Department sent emergency service units and other police personnel, and deployed its aviation unit. Police and rescue workers from around the country took leaves of absence, travelling to New York City to help recover bodies from the remnants of the Twin Towers. Blood donations across the US surged in the weeks that followed. 9/11 was the deadliest incident for firefighters and law enforcement officers in US history, and remains the worst attacks on US soil since Pearl Harbor.

Commemoration ceremonies will be held around the world on Sunday to mark the 15th anniversary of the attacks. This will include an NYPD parade and Tribute in Light, an art installation placed next to the site next to the World Trade Centre, which creates two vertical columns of light. An exhibition entitled "Rendering the Unthinkable: Artists Respond to 9/11." is also showing at the 9/11 Memorial & Museum.