On the morning of Sept. 11, 2001, two hijacked commercial planes crashed into the World Trade Center towers in New York City. Thousands of people died, and the attack helped catapult the United States into its longest war. Twenty years later, the country is still reflecting on the chaos and heartbreak of that day.

See photos of the day’s tragic events below. Some images may be disturbing.

Above: The second tower of the World Trade Center explodes into flames after being hit by an airplane in New York on Sept. 11, 2001.

Hijacked United Airlines Flight 175 flies toward the World Trade Center twin towers shortly before slamming into the south tower (left) as the north tower burns following an earlier attack by a hijacked airliner in New York City on Sept. 11, 2001.

The World Trade Center south tower (left) bursts into flames after being struck by hijacked United Airlines Flight 175 as the north tower burns on Sept. 11, 2001.

People run away as the north tower of the World Trade Center collapses after a hijacked airliner hit the building on Sept. 11, 2001, in New York City.

The top stories of the World Trade Center collapse after being hit by two planes during the Sept. 11 attacks in New York City.

The north tower of New York’s World Trade Center shows the impact left by a hijacked Boeing 767, American Airlines Flight 11, on Sept. 11, 2001. A person is just visible, standing at the bottom of the gaping hole.

A person falls to his death from the World Trade Center after two planes hit the twin towers on Sept. 11, 2001, in New York City.

Hijacked United Airlines Flight 175 from Boston crashes into the south tower of the World Trade Center and explodes at 9:03 a.m. EST on Sept. 11, 2001, in New York City.

A hijacked commercial plane crashes into the World Trade Center on Sept. 11, 2001, in New York.

The twin towers of the World Trade Center burn behind the Empire State Building in New York on Sept. 11, 2001.

The south tower of New York’s World Trade Center collapses on Sept. 11, 2001.

Pedestrians run from the scene as one of the World Trade Center towers collapses on Sept. 11, 2001, in New York.

A lone firefighter moves through piles of debris at the site of the World Trade Center in New York on Sept. 11, 2001.

Rescue workers carry the fatally injured Rev. Mychal Judge, who served as the New York City Fire Department chaplain, from one of the World Trade Center towers on Sept. 11, 2001.

A New York City firefighter carries a water hose on Vessey and Greenwich streets in lower Manhattan after the World Trade Center towers collapsed on Sept. 11, 2001.

A cloud of smoke engulfs the sky and people run for their lives as one of the twin towers collapses on Sept. 11, 2001.

A survivor sits outside the World Trade Center after two planes hit the twin towers on Sept. 11, 2001, in New York City.

Smoke emits from the twin towers of the World Trade Center after they were hit by two hijacked airliners in an attack on Sept. 11, 2001, in New York City.

People in the street watch a building burn in the area where the World Trade Center collapsed on Sept. 11, 2001.

An ambulance covered in debris is on fire after the collapse of the first World Trade Center tower on Sept. 11, 2001, in New York.

New York City firefighters and other emergency personnel survey the World Trade Center area near Vessey and Greenwich streets after the towers collapsed on Sept. 11, 2001.

A man with a fire extinguisher walks through rubble after the collapse of the first World Trade Center tower on Sept. 11, 2001, in New York. The man was shouting as he walked around, looking for survivors who needed assistance.

Rescue workers survey damage to the World Trade Center on Sept. 11, 2001, in New York.

Deputy U.S. Marshal Dominic Guadagnoli helps a woman after she was injured in the attack on the World Trade Center in New York on Sept. 11, 2001.

An American flag at ground zero on the evening of Sept. 11, 2001, after the attack on the World Trade Center in New York City.

Smoke comes out from the Southwest E-Ring of the Pentagon building on Sept. 11, 2001, in Arlington, Virginia, after a hijacked plane crashed into the building and set off a huge explosion.

Firefighters struggle to contain a spreading fire after a highjacked commercial plane crashed into the Pentagon on Sept. 11, 2001, in Arlington, Virginia.

Medical personnel and volunteers help injured people outside the Pentagon after a hijacked commercial airliner crashed into the southwest corner of the building on Sept. 11, 2001.

Military personnel walk down Highway I-395 on Sept. 11, 2001, after evacuating the Pentagon.

A mound of dirt and charred trees mark the site of a crater created by the impact of United Airlines Flight 93, which crashed on Sept. 11, 2001.

Officials examine a crater on Sept. 11, 2001, at the crash site of United Airlines Flight 93 near Shanksville, Pennsylvania.

Officials oversee the scene at the crash site of United Airlines Flight 93 in a field near Shanksville, Pennsylvania, on Sept. 11, 2001.

This article originally appeared on HuffPost and has been updated.