"It is our shared promise — made 25 years ago — to never forget so those born after 9/11 learn its significance and enduring legacy."”

— 9/11 Memorial and Museum

Here are some numbers related to the most deadly attack to occur on U.S. soil, which struck the morning of Sept. 11, 2001, 25 years ago.

19 terrorists who were members of al-Qaeda, an Islamist extremist network, hijacked four commercial airplanes.

8:46 a.m. —  North Tower of World Trade Center attacked by hijackers on American Airlines Flight 11

9:03 a.m. —  South Tower attacked by hijackers on United Airlines Flight 175

9:37 a.m. —  Pentagon attacked by hijackers on American Airlines Flight 77

10:03 a.m. —  Flight 93 crashes in a field near Shanksville, Pennsylvania, after passengers and crew fight back against hijackers

40 passengers and crew aboard Flight 93, which crashed near Shanksville, Pennsylvania; these heroes thwarted another attack that day but lost their lives.

37 phone calls were made by 13 persons on board Flight 93 between the time of the hijacking at 9:28 a.m. and the time of the crash at 10:03 a.m.

2,977 people from more than 90 nations were killed during the terrorist attacks. According to the official death toll, 2,753 people were killed in New York, 184 at the Pentagon and 40 in Pennsylvania.

4,546 civil aircraft were flying in U.S. airspace when the FAA ordered them to land that day. It took about 2.5 hours to clear the airspace, and flights did not resume until two days later, Sept. 13.

May 30, 2002  —  Date of the last column ceremony marking the end of cleanup operation at Ground Zero. Workers removed the final intact column from the site that day. 1.8 million tons of debris was removed from World Trade Center site over 9 months.

World Trade Center complex was home to more than 430 companies from 28 countries.

Oct. 7, 2001 —  Operation Enduring Freedom begins after the Taliban refuse to turn over the mastermind of the attacks, Osama bin Laden.

$25 million —  The bounty issued for information leading to the killing or capture of al-Qaeda leader Osama bin Laden.

May 2, 2011 —  U.S. Navy SEALs assaulted bin Laden's compound in Pakistan and shot and killed him under orders from U.S. President Barack Obama.

Nov. 3, 2014 —  One World Trade Center, a 1,776-foot skyscraper, opens in New York City.

National September 11 Memorial and Museum includes a surviving retaining wall of the World Trade Center with the"Last Column," a 36-foot steel beam to which workers and others attached messages and posters during Ground Zero cleanup operations. A ramp at the museum features a multimedia exhibit with recorded reminiscences by people from 43 countries in 28 languages.

More than 5,700 participants in the World Trade Center Health Program died by 2023; program treated and monitored survivors of the attack for illnesses caused by the attacks.

Dec. 28, 2014  —  Operation Enduring Freedom ends.

"We did not ask for this mission, but we will fulfill it. The name of today's military operation is Enduring Freedom. We defend not only our precious freedoms, but also the freedom of people everywhere to live and raise their children free from fear." — President George W. Bush in address to the nation the afternoon of Oct. 7, 2001

100 million —  Number of Americans who are too young to remember the attacks.

Sources: 9/11 Memorial and Museum, National Park Service, Britannica, George W. Bush White House Archives, Naval History and Heritage Command