The September 11 attacks unfolded on the morning of Tuesday, September 11, 2001, and remain the deadliest terrorist attack ever carried out worldwide.

Nineteen Islamic al-Qaeda operatives hijacked four commercial aircraft within a window of just a few hours.

All four planes had been scheduled for long transcontinental flights to California, which meant their fuel tanks were heavily loaded. That fuel made the resulting impacts and fires dramatically more destructive than they otherwise would have been.

American Airlines Flight 11 was the first to strike, hitting the North Tower of the World Trade Center at 8:46 in the morning.

United Airlines Flight 175 hit the South Tower seventeen minutes later. Millions of people witnessed the second collision as it happened, watching live television coverage that had been focused on the burning North Tower.

Despite being struck second, the South Tower fell first. The location and severity of the damage it sustained caused its structure to fail sooner.

The North Tower held for more than a hundred minutes after impact before it too came down.

American Airlines Flight 77 crashed into the Pentagon, tearing through one side of the largest office building in the world.

United Airlines Flight 93 never reached its intended target. Passengers learned through phone calls what had already happened that morning and made the decision to fight back against the hijackers. The plane went down in a field near Shanksville, Pennsylvania. Investigators concluded that Flight 93 was most likely headed toward the White House or the United States Capitol.

The total number of victims killed in the attacks was 2,977, a figure that does not include the nineteen hijackers. People from more than ninety countries were among those killed, giving the tragedy a genuinely global dimension.

The World Trade Center complex comprised seven buildings in total. Every one of them was either destroyed outright or collapsed as a consequence of the attacks.

7 World Trade Center came down late that same afternoon, though no aircraft had struck it. The building collapsed because of fires and structural damage caused by debris from the collapsing towers.

The Federal Aviation Administration issued an order grounding every civilian aircraft in American airspace, directing all planes to land as quickly as possible. It was the first time such a nationwide grounding had ever been ordered.

Hundreds of international flights that had been approaching North America were diverted to Canada in an emergency operation known as Yellow Ribbon, which saw communities across the country take in stranded passengers.

More than 340 New York City firefighters were killed responding to the attacks. Hundreds of police officers and emergency medical personnel also lost their lives while attempting to rescue people from the buildings.

Cleanup operations at Ground Zero continued for roughly eight months and were completed in May 2002.

The damaged section of the Pentagon was repaired and returned to use within a year.

One World Trade Center, built on the site where the Twin Towers once stood, officially opened in 2014 as the tallest building on the campus.

The National September 11 Memorial incorporates two enormous reflecting pools positioned within the exact footprints left by the fallen towers. The name of every victim of the attacks is inscribed around the edges of those memorial pools.

The attacks prompted the United States to launch what it called the War on Terror, beginning with the invasion of Afghanistan in October 2001.

Forensic experts are still working to identify victims more than two decades later, relying on advances in DNA technology to make identifications that were not possible in the immediate aftermath.

Thousands of first responders who worked at Ground Zero subsequently developed serious illnesses connected to the toxic dust and debris they were exposed to during the cleanup. Many have since died from those conditions.

Each year on September 11, moments of silence are observed at the precise times when each plane struck its target or went down and when each tower collapsed, marking those moments as a way of honoring the victims and the people who responded to the disaster.

September 11 carries a weight that has effectively buried other events that share the date. Among them are William Wallace’s defeat of English forces in 1297, Henry Hudson’s exploration of the river that now bears his name in 1609, Alexander Hamilton being confirmed as the first Secretary of the Treasury in 1789, the disappearance of the Hope Diamond in 1792, and the groundbreaking for the Pentagon in 1941.

Three well-known figures came extraordinarily close to dying on September 11, 2001, each spared by the kind of mundane, accidental circumstance that is almost impossible to believe in retrospect.

Seth MacFarlane, the creator of Family Guy, had a confirmed booking on American Airlines Flight 11, the first plane to hit the World Trade Center. A combination of a hangover and a miscommunication between him and his travel agent about the departure time resulted in him arriving at Boston’s Logan Airport fifteen minutes after the gate had already closed. The plane left without him.

Mark Wahlberg had also been booked on that same flight. A week earlier, however, he had rearranged his plans and chartered a separate aircraft to Toronto instead. He did not learn what had happened until after he had already landed.

Michael Jackson had performed at Madison Square Garden the previous evening and was scheduled for a morning meeting at the top of the World Trade Center on September 11. He stayed up late talking with his mother and sister, overslept, and missed the appointment entirely.