Seventeen years ago, this nation was attacked.
Thousands of lives were lost.
Millions of lives were changed. Forever.
On Tuesday, family and friends gathered to pay tribute to the fathers, sons, brothers, mothers, sisters, and daughters who died on Sept. 11, 2001.
Many gathered for a ceremony where the Twin Towers once stood in New York City. One by one, people read the names of those who died during one of the deadliest days in American history.
Watch: Family and friends gathered for a commemoration at the World Trade Center Memorial in New York City.
On Sept. 11, 2001, four coordinated terrorist attacks against the U.S. killed 2,977 men, women, and children in New York City, Washington, D.C., and Shanksville, Pennsylvania.
Nineteen terrorists hijacked four commercial airliners.
A timeline of the day
8:46 a.m.: American Airlines Flight 11 was flown into New York City's World Trade Center's North Tower. Eleven crew members and 81 passengers died.
9:03 a.m.: United Airlines 175 struck the World Trade Center's South Tower. Two pilots, seven flight attendants, and 56 were killed.
9:37 a.m.: American Airlines flight 77 crashed into the Pentagon in Washington, D.C., killing 184 people.
9:59 a.m.: The South Tower collapsed.
10:03 a.m.: United Airlines Flight 93 crashed into a field near Shanksville, Pennsylvania. The plane crashed after the passengers and crew members attempted to retake control of the plane from the hijackers. All 40 passengers and crew members aboard the flight died. The flight was intended to hit the White House.
10:28 a.m.: The North Tower collapsed.
The death toll included 343 NYC firefighters, 23 NYC police officers, and 37 Port Authority officers who perished while making heroic attempts to rescue those trapped inside the towers.
A nation remembers:
Remembering and honoring, always. #NeverForget https://t.co/uQ5sSQ4gJV— United Airlines (@United Airlines) 1536674404.0
Sept. 11, 2001 – we will never forget. https://t.co/lAaY3IkhGm— americanair (@americanair) 1536669967.0
Powerful moment as the Patriot Flag was displayed during the 9/11 ceremony in @GrapevineTXCity. It has flown at 29… https://t.co/iWkoTIZyQ0— Brandon Todd (@Brandon Todd) 1536674039.0
Watch: President Donald Trump speaking at a memorial ceremony in Shanksville, Pennsylvania.
9:37 a.m., the moment of impact at the Pentagon, where 184 lives were lost. #September11th #NeverForget https://t.co/WmZbySzqeo— Arlington County (@Arlington County) 1536673020.0
Watch: Vice President Mike Pence speaking at a memorial ceremony at the Pentagon Tuesday morning.
LIVE: #SecDef Mattis & Vice Chairman @TheJointStaff #GenSelva host @VP at #Pentagon 9/11 ceremony. #NeverForget https://t.co/efmdtZL3e6— Department of Defense 🇺🇸 (@Department of Defense 🇺🇸) 1536671411.0
Watch: Service members walked 91.1 miles from the Joint Base McGuire-Dix-Lakehurst to the World Trade Center Memorial in New York City.
Candles were lit for each of the men and women who were on United Airlines Flight 93 in Shanksville, Pennsylvania.
Today we honor the brave men and women lost on 9/11. We will #NeverForget them. Lanterns line the Wall of Names… https://t.co/eGSrjNbawE— US Department of the Interior (@US Department of the Interior) 1536675811.0
Former President George W. Bush held a moment of silence at The Bush Center in Dallas, Texas.
“This is a day I will certainly never forget. This morning we pause to say a prayer for the lives lost.” -President… https://t.co/rQ1NAQ4zbP— George W. Bush Presidential Center (@George W. Bush Presidential Center) 1536672358.0
Firefighters across the U.S. participated in annual memorial stair climbs to remember the fallen.
NEVER FORGET- 2018 Wichita 911 Memorial Stairclimb. https://t.co/IST6eCs8YJ— Sid Newby (@Sid Newby) 1536433334.0
Today we climb for our fallen brothers. We remember them with every step we take. #tricitiesclimb #343… https://t.co/bNLZRSXv9w— Carters Valley Volunteer Fire Department (@Carters Valley Volunteer Fire Department) 1536506340.0
On this day 17 years ago, first responders rallied our city and country in the face of tragedy. Their resolve to pr… https://t.co/UseBw7iGYw— Chief Terence Monahan (@Chief Terence Monahan) 1536664894.0
Today we remember those who lost their lives 17 years ago on #September11th and honor the courageous firefighters &… https://t.co/5UujK9K6Ni— Times Square (@Times Square) 1536675462.0
Sports teams and fans, alike, found ways to remember those who made the ultimate sacrifice that fateful day.
Today we remember and honor those who lost their lives on 9/11. #NeverForget https://t.co/9rtCwwFXzi— Chicago White Sox (@Chicago White Sox) 1536674418.0
Forever. Always. #NeverForget 🇺🇸 https://t.co/5y3dvjqu8B— Tampa Bay Lightning (@Tampa Bay Lightning) 1536672901.0
We will #NeverForget. 🇺🇸 https://t.co/Dpv3sBAKwU— Dallas Cowboys (@Dallas Cowboys) 1536674702.0
Today, and everyday, we remember. #NeverForget https://t.co/KV1B63dws7— Philadelphia 76ers (@Philadelphia 76ers) 1536675204.0
More than 60 athletes laced up their shoes to mark the 17th anniversary of 9/11. The 3-day, multi-state trek began at the Pentagon and ended Tuesday at Ground Zero.
Watch: Athletes remember their loved ones during the 240-mile Promise Run.
Florida State remembers 9/11. #NeverForget https://t.co/uWd2h0lf1C— President John Thrasher (@President John Thrasher) 1536674795.0
We must Never Forget The evil of terror The virtue of freedom The necessity of defense The valor of America’s Fir… https://t.co/A34eUO6qlm— Greg Abbott (@Greg Abbott) 1536673433.0
A flag was placed on the lawn outside the Ohio State House in memory of each of the lives lost.
2,977 flags are displayed on the @OhioStatehouse lawn in remembrance of those lost on September 11, 2001. We will… https://t.co/OFcdUcmq3R— Senator Edna Brown (@Senator Edna Brown) 1536673036.0
Watch: The video is a time lapse of the FAA radar as all flights were diverted and grounded on Sept. 11, 2001. Many remember the eerily quiet skies that day.
9:42 A.M. - September 11th, 2001 The FAA orders all civilian planes in U.S. airspace to land and prohibits departu… https://t.co/VVkIIY86ht— James F. Wagner (@James F. Wagner) 1536673339.0
Children are learning about the day that changed America.
The day the earth stood still. A paper quilt made by 6th graders. #NeverForget https://t.co/p3lrw3hXoI— Chit Chat Chicks Podcast (@Chit Chat Chicks Podcast) 1536672745.0
This photo depicts each person who died.
Remembering those who did not come home on September 11th. #NeverForget https://t.co/joV3g2uqNi— IndivisibleWestchester🇺🇸 (@IndivisibleWestchester🇺🇸) 1536673424.0
In 2011, the Port Authority awarded a 9/11 relic to a nonprofit first responder group.
The 300-pound steel beam sits in a garage in Keansburg, New Jersey, while EMS workers Joseph Walsh and his fellow EMS worker Paul Kennedy try to raise enough money to build a memorial dedicated to first responders.
Last year, they held a groundbreaking for the future memorial planned at Keansburg Bech, across from New York Harbor. They haven't come up with the $100,000 needed to build the memorial, which they hope will be completed by Sept. 11, 2019.
Walsh and Kennedy are accepting monetary and building materials donations. Visit the GoFundMe campaign to make a donation.
Watch: Find out more about the first responders' memorial that's in the works.