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NEW YORK CITY — It’s been 15 years since the Sept. 11, 2001 terrorist attacks that shocked the United States and the world.

The attacks in New York, Washington, D.C. and Shanksville, Pennsylvania killed nearly 3,000 people, injured thousands more and left the entire country in mourning.

While 15 years have passed since the attacks, many of the images from that day and the days to follow remain unforgettable.

Sept. 11, 2001

President Bush's Chief of Staff Andy Card whispers into the ear of the President to give him word of the plane crashes into the World Trade Center, during a visit to the Emma E. Booker Elementary School in Sarasota, Fla., Tuesday, Sept. 11, 2001. (Doug Mills, AP Photo)

Smoke billows from one of the towers of the World Trade Center and flames as debris explodes from the second tower, in this Sept. 11, 2001, file photo. (Chao Soi Cheong, AP Photo)

People hang out of broken windows of the North Tower of the World Trade Center after a terrorist attack in New York on the morning of Sept. 11, 2001. (Amy Sancetta, AP Photo)

A helicopter flies over the Pentagon in Washington, Tuesday, Sept. 11, 2001 as smoke billows over the building. American Airlines Flight 77 crashed into the Pentagon at 9:37 a.m. (Heesoon Yim, AP Photo)

The south tower of the World Trade Center begins to collapse following the terrorist attack on the New York landmark Tuesday, Sept. 11, 2001. The Millenium Hilton hotel is in foreground. (Amy Sancetta, AP Photo)

People run from the collapse of World Trade Center Tower Tuesday, Sept. 11, 2001 in New York. (Suzanne Plunkett, AP Photo)

People run from the collapse of World Trade Center towers in New York, Tuesday, Sept. 11, 2001 after terrorists crashed two hijacked airliners into the World Trade Center and brought down the twin 110-story towers. (Suzanne Plunkett, AP Photo)

Emergency workers look at the crater created when United Airlines Flight 93 crashed near Shanksville, Pa., on Tuesday, Sept. 11, 2001. (Keith Srakocic, AP Photo)

Women wearing dust masks flee across the Brooklyn Bridge from Manhattan to Brooklyn following the collapse of both World Trade Center towers Tuesday, Sept. 11, 2001 in New York. (Mark Lennihan, AP Photo)

President Bush speaks to Vice President Dick Cheney by phone aboard Air Force One after departing Offutt Air Force Base in Nebraska, Tuesday, Sept. 11, 2001. (Eric Draper, AP Photo/The White House)

In this Sept. 11, 2001 file photo, firefighters work beneath the destroyed mullions, the vertical struts which once faced the soaring outer walls of the World Trade Center towers, after a terrorist attack on the twin towers in New York. (Mark Lennihan, AP Photo)

The Days After

On Sept. 12, 2001 and the days after the attack, hundreds of victims were unaccounted for. Boards were filled with photos of missing individuals were placed near where the World Trade Center towers once stood.

A woman looks at missing person posters of victims of the September 11 terrorist attacks on the World Trade Center in New York City on Sept. 14, 2001.(Robert Spencer, AP Photo)

An unidentified woman wipes away tears as she views photographs of missing persons in front of the Armory on 25th Street, which has been converted to a Family Center for victims of the terrorist attack, in New York, Sunday, Sept. 16, 2001. (Charles Dharapak, AP Photo)

Meanwhile, millions across the world joined in with the U.S. in mourning those lost in the attacks. Many also gave thanks to the hundreds of emergency crews who responded to help save lives.