Nearly 3,000 people were killed when al-Qaeda hijackers crashed four jetliners into the Twin Towers, the Pentagon and a field in southwestern Pennsylvania on September 11, 2001.

Below is a summary of the major events that occurred, beginning with the first plane hitting the North Tower, according to the 9/11 Memorial & Museum.

08:46 am

Five hijackers cause American Airlines Flight 11 to crash into floors 93 through 99 of the North Tower. The 76 passengers and 11 crew members on board and hundreds in the building die instantly. Hundreds of people are trapped above the 91st floor because the crash cuts through all three emergency stairwells.

08:50 am

Then President George W. Bush is informed of the plane crash while sitting in a Florida elementary school classroom. He and his advisers assume it was an accident.

08:59 am

The Port Authority orders the evacuation of both towers. A minute later, they evacuate the entire World Trade Center complex.

09:03 am

Five hijackers crash United Airlines Flight 175 into floors 77 through 85 of the South Tower, killing the 51 passengers and nine crew members aboard the plane and an unknown number of people in the building. Many were trapped in elevators or on floors above the impact zone. People can be seen jumping to their deaths from the towers.

The estimated death toll currently ranges from 50 to over 200.

The NYPD is calling for a second Level 4 mobilization.

09:05 am

President Bush learns that the second plane has crashed. He gives a one-minute speech to the public from Florida before he leaves. At this point it is a terrorism investigation.

09:37 am

Five hijackers crash American Airlines Flight 77 into the Pentagon. The 53 passengers and six crew members on board are killed. Another 100 military personnel and civilians on the ground are also killed.

09:59 am

The South Tower collapses in 10 seconds after burning for almost an hour, killing more than 800 people inside the building at the time.

10:03 am

Four hijackers crash Flight 93 into a field near the Pennsylvania town of Shanksville, about a 20-minute flight from Washington, D.C., after passengers and crew stormed the cockpit. All 33 passengers and seven crew members on board are killed.

The tri-state area is commemorating the 23rd anniversary of the September 11, 2001, attacks. NBC New York presents a special report with coverage of ceremonies in Lower Manhattan and across the region.

10:15 am

Part of the Pentagon collapses.

10:37 am

The North Tower collapses. More than 1,600 people die as a result of the attack on that building.

11:02 am

Then-New York City Mayor Rudy Giuliani calls for the evacuation of Lower Manhattan.

Early afternoon

Rescue efforts continue at the World Trade Center site, now known as Ground Zero.

A photographer took a picture of Edward Fine as he walked away from the Twin Towers, covered in dust and debris, with a cloth over his mouth.

5:20 PM

7 World Trade Center collapses after burning for hours. It had been evacuated long before.

8:30 pm

Bush addresses the nation from the White House.

“The search for those behind these evil acts is ongoing,” he said. “I have deployed the full resources of our intelligence and law enforcement communities to find those responsible and bring them to justice. We will not discriminate between the terrorists who committed these acts and those who harbor them.”