For those of us who were around on September 11, 2001, the past 23 years have gone by pretty quickly. But our memory of that day will stay with us forever.

On a crystal clear Tuesday morning, an American Airlines Boeing 767 loaded with 20,000 gallons of jet fuel that had departed from Logan International Airport in Boston at 7:59 a.m. crashed into the north tower of the World Trade Center in New York City at 8:46 a.m.

The impact left a gaping, burning hole in several upper floors of the 110-story skyscraper, instantly killing hundreds of people and trapping hundreds more in the floors above.

As the evacuation of the tower and its twin got underway, television cameras broadcasted live images of what initially appeared to be a freak accident involving Flight 11, which had 81 passengers and 11 crew members aboard and was destined for Los Angeles, not NYC.

Then, 18 minutes after the first plane hit, a second Boeing 767—United Airlines Flight 175—appeared out of the sky, turned sharply toward the World Trade Center and sliced into the south tower. This plane had also departed from Logan International Airport in Boston, at 8:14 a.m. and was destined for Los Angeles with 56 passengers and nine crew members aboard.

The collision of Flight 175 caused a massive explosion that showered burning debris over surrounding buildings and onto the streets below. It immediately became clear that America was under attack.

On that day, 19 militants associated with the Islamic extremist group al Qaeda hijacked four airplanes and carried out suicide attacks against targets in the United States. Two of the planes were flown into the twin towers of the World Trade Center in New York City, a third plane hit the Pentagon just outside Washington, D.C., and the fourth plane crashed in a field in Shanksville, Pennsylvania, after its passengers and crew fought back.

Nearly 3,000 people were killed during the 9/11 terrorist attacks. Among them were innocent citizens of 78 countries who died in New York, Washington, D.C., and Pennsylvania. The actions of 19 terrorist hijackers aboard those four airplanes triggered major U.S. initiatives to combat terrorism and ultimately defined the presidency of George W. Bush.

At the World Trade Center, 2,763 died after the two planes slammed into the twin towers. That figure includes 343 firefighters and paramedics, 23 New York City police officers and 37 Port Authority police officers that were struggling to complete an evacuation of the buildings and save the office workers trapped on higher floors.

At the Pentagon, 189 people were killed, including 64 on American Airlines Flight 77, the airliner that struck the building. The Boeing 757 had departed from Washington, D.C. at 8:20 a.m. and was also headed to Los Angeles.

On United Airlines Flight 93, a total of 44 people died when the plane crash-landed in Shanksville after passengers and crew members rushed the hijackers. That plane, which had departed from Newark, N.J. at 8:42 a.m. en route to San Francisco, California, was believed to have been destined by the hijackers for another landmark building in Washington, D.C. if the passengers and crew members had not taken the heroic actions that they did.

Twenty-three years later our service personnel in the military and others are still fighting a seemingly endless war against terrorism. They do so, in part, because no one ever wants to be on the receiving end of a telephone message such as the one Flight 93 passenger Linda Gronlund left for her sister at 9:46 a.m. on September 11, 2001, according to the 9/11 Commission Report: FBI transcripts of recorded calls: “Apparently, they, uh, flown a couple of planes into the World Trade Center already and it looks like they’re going to take this one down as well…Mostly, I just wanted to say I love you and I’m going to miss you.”

Just 19 minutes later, Flight 93 crashed upside-down in Stonycreek Township at 10:03 a.m. and its impact was immediately felt by the eyewitnesses throughout the Shanksville and Stonycreek region.

The impact of that fateful day continues to reverberate today. We said we’d never forget. Those of us who vividly recall the tragedy that was 9/11 know why we can’t.