9/11 Timeline: Revisiting the September 11 Attacks at World Trade Center, Pentagon, Flight 93

The attacks on September 11, 2001, were a defining moment in millions of lives. Eighteen years later, people still pause to remember the events of that day and their global impact.

Wednesday marked the 18th anniversary of the terrorist attacks that claimed the lives of almost 3,000 people at the World Trade Center, Pentagon and on United Airlines Flight 93. Each year, remembrance services are held in New York City, Washington, D.C. and Shanksville, Pennsylvania, where Flight 93 crashed.

While an unforgettable moment for millions of people, as the years pass, the population of those too young to remember that fateful Tuesday morning grows. Therefore, similarly to other historic events, it becomes increasingly important to remember the circumstances of that day to preserve the memory of its thousands of victims and heroes.

9/11 attacks timeline world trade center
A hijacked commercial plane crashes into the World Trade Center on September 11, 2001, in New York. Wednesday marked the 18th anniversary of the attacks. SETH MCALLISTER/AFP/Getty

Timeline of Events

5:45 a.m.: Hijackers Mohamed Atta and Abdul Aziz al-Omari board a flight at Portland International Jetport in Maine, later connecting to American Airlines Flight 11 at Boston Logan International Airport.

7:49 a.m.: Flight 11 takes off.

8:15 a.m.: United Airlines Flight 175 takes off from Boston, bound for Los Angeles.

8:20 a.m.: American Airlines Flight 77 takes off from Washington Dulles International Airport.

8:24 a.m.: Atta broadcasts a message to air traffic control, "We have some planes. Just stay quiet and you will be okay." The captain of Flight 175, United Airlines pilot Victor J. Saracini, picks up Atta's transmission from Flight 11 and informs the Federal Aviation Administration.

8:37 a.m.: Boston's air traffic control center alerts the Air Force's Northeast Air Defense Sector to Atta's message. Air National Guard jets are mobilized to follow Flight 11.

8:42 a.m.: United Airlines Flight 93 takes off from Newark International Airport.

8:46 a.m.: Flight 11 crashes into floors 93 through 99 of the north tower of the World Trade Center. Responders mobilize.

8:50 a.m.: President George W. Bush is notified of the attacks while visiting an elementary school in Florida.

8:52 a.m.: A flight attendant on Flight 175 informs a United Airlines operator that the flight has been hijacked.

8:59 a.m.: Port Authority Police Department Sergeant Al Devona orders the twin towers to be evacuated.

9:03 a.m.: United Airlines Flight 175 crashes into floors 77 through 85 of the south tower of the World Trade Center.

9:05 a.m.: Bush is informed a second plane hit the south tower.

9:12 a.m.: Passengers on Flight 77 call their loved ones to tell them the plane was hijacked.

9:37 a.m.: American Airlines Flight 77 crashes into the Pentagon.

9:42 a.m.: The Federal Aviation Administration (FAA) grounds all flights.

9:45 a.m.: The White House and Capitol are evacuated.

9:58 a.m.: Flight 93 passenger Edward P. Felt uses his cell phone to dial 9-1-1 and inform emergency operators of the hijacking.

9:59 a.m.: The south tower collapses.

10:03 a.m.: Passengers on United Airlines Flight 93 attack the hijackers aboard their plane in an attempt to seize control. The hijackers crash the plane into a field near Shanksville, Pennsylvania.

10:15 a.m.: The Pentagon's E Ring collapses.

10:28 a.m.: The north tower collapses.

11:02 a.m.: New York City Mayor Rudolph Giuliani orders the evacuation of Lower Manhattan.

12:16 p.m.: United States airspace is closed.

12:30 p.m.: A group of survivors is found in the lower section of the North Tower's stairwell B.

5:20 p.m.: The building at 7 World Trade Center collapses.

8:30 p.m.: Bush addresses the nation from the White House.

At the World Trade Center, 2,753 people were killed. An additional 184 perished at the Pentagon, and 40 were killed when Flight 93 crashed in Pennsylvania. Eighteen years later, first responders are still dealing with the mental and physical effects of clearing the wreckage at ground zero. According to ABC News, more than 200 people have died from illnesses related to the attacks.