The south tower of the World Trade Center collapses.

Policemen and firemen run away from the huge dust cloud caused as the north tower of the World Trade Center collapses.

Marcy Borders is covered in dust as she takes refuge in an office building after one of the World Trade Center towers collapsed in New York. Borders was outside on the street as the cloud of smoke and dust enveloped the area. Borders was diagnosed with stomach cancer in August 2014, which she believed was a side effect of the toxic dust she was exposed to during the 9/11 attacks. She died on August 24, 2015.

A rescue dog is transported out of the debris of the World Trade Center on September 15, 2001.

Mike Scott from the California Task Force-8 and his dog, Billy, search through rubble for victims of the September 11 terrorist attack at the World Trade Center September 21, 2001 New York City, NY.

Civilians bolt in the opposite direction as firefighters rush towards the Twin Towers of the New York City’s World Trade Center after a plane hit the building on September 11, 2001.

Suicide hijackers flew planes into both towers of the World Trade Center, causing the towers’ eventual collapse. The 9/11 attacks not only became the single deadliest terrorist attack in human history, they were also the deadliest incident ever for firefighters, as well as for law enforcement officers in the United States.

The New York City Fire Department lost 343 among their ranks on 9/11. Here, FDNY firefighters react after the collapse of the building.

A New York firefighter is seen alone amid the rubble of the World Trade Center following the attacks. “We had a very strong sense we would lose firefighters and that we were in deep trouble, FDNY Division Chief for Lower Manhattan Peter Hayden later told the 9/11 Commission. “But we had estimates of 25,000 to 50,000 civilians, and we had to try to rescue them.”

Members of the FDNY carry fellow firefighter, Al Fuentes, who was injured in the collapse of the World Trade Center. Captain Fuentes, who had been pinned under a vehicle on the west side highway, survived after his rescue.

A firefighter crouches in grief at the World Trade Center site on 9/11.

The rubble of the World Trade Center smolders on September 12, 2001 as firefighters continue recovery efforts.

On September 14, 2001, President George W. Bush flew to New York City and visited the World Trade Center site. Here the president comforts New York City firefighter, Lt Lenard Phelan of Battalion 46, whose brother, Lt Kenneth Phelan of Battalion 32, was among the 300 members of the FDNY still unaccounted following the attacks. Kenneth Phelan was eventually identified among the firefighters killed.

An estimated 17,400 people had been at the World Trade Center on the day of the 9/11 attacks, and some 87 percent of them were safely evacuated thanks in large part to firefighters’ heroic efforts.

In this handout provided by the Federal Bureau of Investigation, first responders on scene following an attack at the Pentagon on September 11, 2001 in Arlington, Virginia. American Airlines Flight 77 was hijacked by al Qaeda terrorists who flew it in to the building killing 184 people.

First responders pour water on the fire on scene following the attacks.

This FBI photo shows a closer look at the damage to the building.

Emergency workers and firefighters worked through the night searching for survivors.

A piece of debris from American Airlines Flight 77 that was collected by the FBI on scene following the attacks.

Another piece of debris from American Airlines Flight 77 that was collected by the FBI on scene following the attacks.

Smoke rises behind investigators as they comb the crater left by the crash of United Airlines flight 93 near Shanksville, Pennsylvania September 12, 2001. Flight 93 is one of four planes that were hijacked as part of a deadly and destructive terrorist plot against the U.S. September 11.

A yellow crime scene tape lays discarded next to a cross draped with white cloth that was erected on a hill overlooking the once peaceful valley where United Flight 93 crashed, killing 38 passengers and seven crew members. This photo was taken on September 24, 2001 as charred trees and piles of dirt still remain as reminders of the fateful day. Power lines and paved roads were installed by the investigators in the rural setting.

This photograph released by the U.S. District Court shows the flight data recorder found at the scene where United Flight 93 crashed.

Amy Shumaker, of Hooversville, PA, holds her son Ryan Shumaker, 4, at the Flight 93 memorial near Shanksville, Pennsylvania, on September 4, 2002. Shumaker said she was one of the first EMT’s on the scene at the time of the crash.

On September 24, 2002, Congress passed the Flight 93 National Memorial Act. The Act created a new national park unit to commemorate the passengers and crew of Flight 93 which opened to the public in 2015. The Flight 93 National Memorial is pictured here on September 10, 2016 in Shanksville, Pennsylvania.

President George W. Bush reacts as he’s informed by his chief of staff, Andrew Card, of the attacks on the World Trade Center in New York. The president had been reading to a second grade class during an early morning visit to an elementary school in Sarasota, Florida on September 11, 2001.

As Dan Bartlett, deputy assistant to the president, points to news footage of the World Trade Center, President Bush gathers information about the terrorist attack. Also pictured in a classroom at Emma E. Booker Elementary School in Sarasota are, from left: Deborah Loewer, director of White House Situation Room, and Senior Adviser Karl Rove.

U.S. Secret Service and Military Police go on high alert and double up security checks for all passengers boarding Air Force One 11 as President Bush departs Sarasota.

Back at the White House, Vice President Dick Cheney watches the news of the attacks on the World Trade Center in his office before being led to the underground White House bunker.

Vice President Cheney with senior staff in the Presidential Emergency Operations Center (PEOC), the Cold War-era bunker under the White House.

President Bush speaks to Vice President Cheney by phone aboard Air Force One on September 11, 2001 after departing Offutt Air Force Base in Sarpy County, Nebraska.

President Bush watches television coverage of the terrorist attacks on the World Trade Center on September 11, 2001, from his office aboard Air Force One.

President Bush talks on the telephone as senior staff huddle in his office aboard Air Force One. Not knowing how many more hijacked planes might be heading for the nation’s capital, the Secret Service decided it would be unsafe to take the president back to Washington.

President Bush and his staff look out the windows of Air Force One at their F-16 escort on September 11, 2001, while en route to Barksdale Air Force Base in Louisiana. Initially, they weren’t sure whether the approaching planes were hostile. Pictured from left are: Andy Card, White House chief of staff; Ari Fleischer, press secretary; Blake Gottesman, personal aide to the president; Karl Rove, senior adviser; Deborah Loewer, director of White House Situation Room, and Dan Bartlett, deputy assistant to the president.

An F-16 escorts Air Force One from Offutt Air Force Base in Nebraska back to the nation’s capital on September 11, 2001.

This American Airlines flight attendant wings lapel pin belonged to Karyn Ramsey, friend and colleague of 28-year-old Sara Elizabeth Low, who was working aboard Flight 11, which crashed into the North Tower of the World Trade Center. Following the memorial service for Sara, Karyn pinned her own service wing on Sara’s father, Mike Low. Mike Low would refer to the lapel pin as “Karyn’s wings.” Watch this video to learn more.

This pager, recovered from Ground Zero, belonged to Andrea Lyn Haberman. Haberman was from Chicago and was in New York City on September 11, 2001 for a meeting at Carr Futures offices, located on the 92nd floor of the North Tower. It was Haberman’s first time visiting New York; she was only 25 years old when she was killed in the attacks.

On the morning of September 11, 55-year-old Robert Joseph Gschaar was working on the 92nd floor of the South Tower. At the time of the attack, he called his wife to let her know about the incident and reassured her that he would safely evacuate. Robert did not make it out of the tower alive. A year after the attacks his wallet and wedding ring were recovered.Inside his wallet was a $2 bill. Robert and his wife, Myrta, carried around $2 bills during there 11-year marriage to remind each other that they were two of a kind.

On September 11, FDNY Squad 18 responded to the attacks on the Twin Towers. Among this unit was David Halderman, who was a firefighter just like his father and brother. His helmet was found crushed on September 12, 2001 and given to his brother, Michael, who believes his death was due to the the collapse of the tower and a strike to the head. David Halderman’s body was not recovered until October 25, 2001.

This I.D. card belonged to Abraham J. Zelmanowitz, an Empire BlueCross BlueShield computer programmer. On the morning of the attacks, he was working on the 27th floor of the North Tower, along with a wheelchair-bound friend, Edward Beyea. Zelmanowitz decided to stay behind to remain by his friend’s side as the rest of the company began to evacuate. Coworkers who evacuated informed professional emergency responders that the two were awaiting assistance inside.FDNY Captain William Francis Burke, Jr. arrived at the scene on the 27th floor as the South Tower began to collapse. Burke, with the same bravery as Zelmanowitz, sacrificed his life to help others by telling his team to evacuate to safety while he stayed behind to try and help Zelmanowitz and Beyea. The three men would only make it as far down as the 21st floor, making phone calls to loved ones before their deaths.

This pair of women’s heels belonged to Fiduciary Trust employee Linda Raisch-Lopez, a survivor of the attacks on the World Trade Center. She began her evacuation from the 97th floor of the South Tower after seeing flames from the North Tower. She removed her shoes and carried them as she headed down the stairs, reaching the 67th floor when the South Tower was stuck by Flight 175. As she headed uptown to escape, she put her shoes back on, and they became bloody from her cut and blistered feet. She donated her shoes to the museum.

This gold link bracelet belonged to Yvette Nicole Moreno. Bronx native Yvette Nicole Moreno was working as a receptionist at Carr Futures on the 92nd floor of the North Tower, after recently being promoted from a temporary position. After the North Tower was hit, she called her mother to let her know she was heading home. However, on her way out of the office she was struck by debris from the South Tower, dying at the young age of 24.

This baseball cap belonged to 22-year veteran of the Port Authority Police Department, James Francis Lynch. At the time of the attacks, James was off duty and recovering from surgery, but felt the need to respond. He had previously responded to the 1993 bombing of the World Trade Center. He died at the age of 47 that day, and his body was not recovered until December 7, 2001.

This police badge belonged to John William Perry, a New York Police Department officer with the 40th Precinct and a N.Y. State Guard first lieutenant. He was another off-duty officer who responded to the attacks. He had plans to retire from the police force to pursue a career as a full-time lawyer. He was 38 years old.

On March 30, 2002 a firefighter working at Ground Zero found a bible fused to a piece of metal. The bible was open to a page with fragments of legible text reading “an eye for an eye” and “resist not evil: but whosoever shall smite thee on thy right cheek, turn to him the other also.” Watch this video to learn more about the bible.