For Americans and people watching around the world, September 11, 2001, is a day that will never be forgotten.

Within three hours, New York’s tallest buildings were reduced to rubble, and the Pentagon—the nerve center of the American armed forces—was burning and partially collapsed. Thousands of civilians had lost their lives and were seriously injured, and the entire country was in collective shock, still trying to make sense of how a coordinated act of terrorism of that magnitude was allowed to take place on American soil.

In the 20 years since 9/11, the events that occurred that morning have been analyzed in-depth from a thousand different angles. Even though the attacks took place in the era just before mobile phones had viable cameras, there are countless images and videos of the event. As well, we now have the 9/11 Commission Report, which compiles interviews from over 1,200 people in 10 countries, and draws upon two and a half million pages of documents to present its findings.

For many people younger than Generation X, 9/11 is a feeling—a grim milestone from their youth—but the details are likely more fuzzy. The timeline visualization above is a high-level record of what happened that morning during the three hours when everything changed.

In its most simple form, the 9/11 attacks can be described as a coordinated hijacking of four commercial airplanes, which were then used to fly into high profile targets in New York City and Washington, DC. Here is a summary of the planes involved in the incident:

These four flights play a central role in what unfolded that morning. In the early hours of September 11, 2001, a collection of 19 would-be hijackers made their way through security at airports in Boston, Newark, and Washington, DC.

Our three-hour timeline begins just before 8am, as the first plane involved in the attack leaves the tarmac just outside of Boston. (In situations where the exact time isn’t known, a range is given.)

Sept 11, 2001, 7:59am – American Airlines Flight 11, a Boeing 767 carrying 81 passengers and 11 crew members, departs from Logan International Airport in Boston, bound for Los Angeles International Airport.

8:14 – United Airlines Flight 175, a Boeing 767, carrying 56 passengers and 9 crew members, departs from Logan International Airport in Boston, bound for Los Angeles International Airport.

8:14 – Flight 11 is hijacked over central Massachusetts. There are five hijackers on board.

8:20 – American Airlines Flight 77, a Boeing 757 with 58 passengers and 6 crew members, departs from Washington Dulles International Airport, for Los Angeles International Airport.

8:42 – United Airlines Flight 93, a Boeing 757 with 37 passengers and 7 crew members, departs from Newark International Airport, bound for San Francisco International Airport.

8:42–8:46 – Flight 175 is hijacked above northwest New Jersey. There are five hijackers on board.

8:46 – Flight 11 crashes into the north face of the North Tower (1 WTC) of the World Trade Center, between floors 93 and 99. All 92 people on board are killed.

8:50–8:54 – Flight 77 is hijacked above southern Ohio. There are five hijackers on board.

9:03 – Flight 175 crashes into the south face of the South Tower (2 WTC) of the World Trade Center, between floors 77 and 85. All 65 people on board are killed.

9:28 – Flight 93 is hijacked above northern Ohio. There are four hijackers on board.

9:37 – Flight 77 crashes into the western side of The Pentagon. All 64 people on board are killed.

9:45 – United States airspace is shut down; all operating aircraft are ordered to land at the nearest airport.

9:59 – The South Tower of the World Trade Center collapses, 56 minutes after the impact of Flight 175.

10:03 – Flight 93 is crashed by its hijackers in a field in Somerset County, Pennsylvania. Later reports indicate that passengers had learned about the World Trade Center and Pentagon crashes and were resisting the hijackers. All 44 people on board are killed in the crash.

10:28 – The North Tower of the World Trade Center collapses, 1 hour and 42 minutes after the impact of Flight 11. The Marriott Hotel at the base of the two towers is also destroyed.

10:50 – Five stories of the western side of the Pentagon collapse due to the fire.

Two and a half hours after the first plane left Boston, the iconic “Twin Towers” lay in ruins in Lower Manhattan, and brave first responders and military personnel were scrambling to save lives and secure the country.

Life in America was set on a new trajectory.

Two decades is a long time in the world of technology and media. Though the communication channels of that era may seem slow by today’s standards, the September 11 terrorist attacks still took place in the age of 24-hour cable news coverage and nascent online reporting.

Add in the fact that New York was (and still is) a linchpin of global media, and it’s easy to see why media coverage of the attack spread so quickly.

Within two minutes of the first impact on the World Trade Center, a nearby camera crew covering New York’s mayoral primary election was already broadcasting a live feed of the burning building to a TV audience. Within three minutes, news of the attack hit the Associated Press newswire, and moments after that, most major networks cut away from scheduled programming to cover the story.

Less than 10 minutes after the impact, President Bush–who was attending an event at a Florida elementary school–was informed of the crash (which at that point was characterized as an accident).

Because media outlets were able to cover the incident so quickly, millions of people witnessed the second plane striking the South Tower in real-time a mere 17 minutes after the first impact. This was a defining moment as millions of people around the world experience the events precisely as they unfolded.

The still-young internet was strained that day. Moments after the impact of the North Tower, the CNN and MSNBC websites experienced a crushing load of traffic that overwhelmed servers. The FBI’s website also experienced issues after posting the images of the 9/11 hijackers later that day.