The country was forever changed on this day 21 years ago.

Terrorists hijacked and crashed passenger planes into buildings in New York and just outside of Washington, D.C. One hijacked plane dove into an empty field, its hijackers’ plans foiled by a group of courageous passengers. 

Nearly 3,000 victims’ lives were claimed by extremist group al-Qaida’s 9/11 attacks. New York City lost more than 400 first responders who rushed to help at the burning World Trade Center. More than 83,000 first responders have enrolled in a Centers for Disease Control program that provides support for a number of medical conditions, like cancer and post-traumatic stress disorder, that arose from being at the site of the attacks. 

From the archives:As 9/11 attacks unfolded, no one knew if hijackers were on board

Sept. 11, 2001, set the stage for a new “global war on terror” launched by former U.S. President George W. Bush. The United States military and its allies set out to eliminate al-Qaida terrorist network harbored by the Taliban in Afghanistan in a costly war that ended last year with the United States withdrawing its troops and the Taliban taking control of the county. 

American counterterrorism operations have eliminated minds behind the 9/11 attacks. A 2011 Navy SEAL operation killed the infamous al-Qaida leader, Osama bin Laden, and a missile strike in July killed Ayman al-Zawahri, who the United States considered a co-planner of the 9/11 attacks. 

Opinion:War on terror moved my family to a Muslim country. It was an adventure of a lifetime.

Here are some of most powerful images captured during and after the 9/11 attacks. 

In this Sept. 11, 2001 file photo, United Airlines Flight 175 collides into the south tower of the World Trade Center in New York as smoke billows from the north tower.

A man coated with ash and debris from the collapse of the World Trade Center south tower coughs near City Hall in lower Manhattan Sept. 11, 2001.

Two women hold each other as they watch the World Trade Center burn following a terrorist attack on the twin towers in New York, in this Sept. 11, 2001 file photo.

Military personnel flee from the Pentagon after a plane crashed into the building Tuesday morning, Sept. 11, 2001 in Washington.

The remains of the World Trade Center stand amid other debris Sept. 11, 2001, following the terrorist attack on the buildings in New York.

People flee lower Manhattan across the Brooklyn Bridge in New York, Tuesday, Sept. 11, 2001, following a terrorist attack on the World Trade Center.

In this file photo investigative personnel search the crash site of United Airlines Flight 93 looking for debris and evidence, including the plane's flight recorder, in Shanksville, Pennsylvania on September 12, 2001.

In this September 13, 2001 photograph, people gather around missing person posters on a telephone booth after the September 11 terrorist attacks on the World Trade Center in New York City.

President Bush talks with Chief of Staff Andrew Card aboard Air Force One during a flight to Offutt Air Force Base in Omaha, Neb., following the presidents' statement about the terrorist attack on the World Trade Center in New York City, Tuesday, Sept. 11, 2001.

A monitor reads all flights cancelled departing Greenville-Spartanburg International Airport in South Carolina on September 12, 2001. Airplanes were grounded around the country.

In this Sept. 11, 2001 file photo the twin towers of the World Trade Center burn behind the Empire State Building in New York after terrorists crashed two planes into the towers causing both to collapse.

Justin Eckert comforts his wife, Dorinda, at the American Airlines ticket counter at Portland International Airport as she feeds their 3-month-old son, Ethan, in Portland, Ore., Tuesday, Sept. 11, 2001, after finding out their flight to Missouri has been canceled due to terrorist attacks on the New York World Trade Center towers and the Pentagon.

Seniors at Murphy High School, in Mobile, Ala., watch news reports Tuesday Sept. 11, 2001, about the terrorist attacks at the World Trade Center in New York City.

From the archives:‘Clear the skies’: Behind the unprecedented call to stop air travel on 9/11