Today marks 25 years since the September 11, 2001 terrorist attacks, a day that changed the United States forever and claimed the lives of 2,977 people.

Across America today, ceremonies and moments of silence will remember those killed at the World Trade Center in New York City, the Pentagon in Arlington, Virginia, and aboard United Airlines Flight 93, which crashed near Shanksville, Pennsylvania.

It was a clear Tuesday morning when four commercial airliners were hijacked.

At 8:46 AM, American Airlines Flight 11 struck the North Tower of the World Trade Center. At 9:03 AM, United Airlines Flight 175 struck the South Tower.

American Airlines Flight 77 struck the Pentagon at 9:37 AM, and at 10:03 AM, United Airlines Flight 93 crashed in Pennsylvania after passengers and crew members fought back against the hijackers.

Both World Trade Center towers collapsed that morning. By the end of the attacks, 2,977 victims had been killed, including hundreds of firefighters, police officers and other first responders.

September 11 is now observed each year as Patriot Day. For today’s 25th anniversary, flags at federal buildings are being flown at half-staff in honor of those killed.

At the National September 11 Memorial in New York, family members of victims will gather this morning to read aloud the names of those killed in the 2001 attacks and the 1993 World Trade Center bombing.

This year’s ceremony will include seven moments of silence. Six correspond to the moments when the towers were struck and fell, the attack on the Pentagon and the crash of Flight 93. A seventh has been added this year to remember those who have died from illnesses and injuries connected to 9/11 and its aftermath.

The 25th anniversary also marks a major generational change in how America remembers September 11.

The National September 11 Memorial & Museum says approximately 100 million Americans are now either too young to remember the attacks or were born afterward. That has placed an increasing emphasis on preserving firsthand accounts and teaching younger generations what happened that morning.

Tonight, the Tribute in Light will once again shine above Lower Manhattan. The two beams of light represent the Twin Towers and have become one of the most recognizable symbols of remembrance associated with September 11.

For Americans old enough to remember that Tuesday morning, September 11 remains one of those rare moments when people can still recall where they were and what they were doing when they first learned the nation was under attack.

Twenty-five years later, America pauses again today to remember the victims, honor the first responders and others who answered the call, and carry forward the promise made in the aftermath of the attacks:

Never Forget.