Smoke and debris fill the streets as pedestrians run for cover after the collapse of the south tower. What started as a bright sunny day turned to darkness. "Suddenly the top of [the tower] just shattered into tens of thousands of pieces," said Steve Johnson of msnbc.com. "You could see the whole thing just disappeared. Then the smoke came up. The cops started yelling, 'Get back! Run! Get Away!' I ran inside a hotel, and it went black outside because of the dust."

Remembering 9/11

20 years ago: Looking back at 9/11 in pictures

Nearly 3,000 people lost their lives on Sept. 11, 2001, in coordinated attacks on the World Trade Center and Pentagon and a crash in Shanksville, Pennsylvania.

People in front of New York's St. Patrick's Cathedral react with horror as they look down Fifth Ave towards the World Trade Center towers after planes crashed into their upper floors.

People in front of New York's St. Patrick's Cathedral react as they look down Fifth Ave towards the World Trade Center towers after planes crashed into their upper floors.

Eds. note: This gallery contains graphic images that may be disturbing.

— Marty Lederhandler / AP

On Sept. 11, 2001, hijackers caught U.S. authorities by surprise with a bold attack on New York City's World Trade Center. At 8:46 a.m., American Airlines Flight 11 carrying 92 people, speared into the 110-story north tower. At 9:03 a.m., United Airlines Flight 175, shown here, carrying 65 people, crashed into the 84th floor of the south tower.

— Sean Adair / Reuters

President Bush is interrupted at 9:07 a.m. during a school visit in Sarasota, Fla., by Andrew Card, his chief of staff, and informed that a second plane had hit the World Trade Center.

Chief of staff Andrew Card interrupts President Bush at 9:07 a.m. during a school visit in Sarasota, Fla., to inform him that a second plane hit the World Trade Center. Bush left the school session and set off on a secretive hopscotch flight aboard Air Force One, stopping at an Air Force Base in Louisiana and NORAD headquarters in Nebraska before returning to the White House late in the afternoon.

— Paul J. Richards / AFP - Getty Images

A plane hits the south tower of the World Trade Center.

— Spencer Platt / Getty Images

With the White House in the foreground, the Pentagon burns on Sept. 11, 2001, after an airplane crashed into the U.S. military headquarters.

With the White House in the foreground, smoke billows from the Pentagon on Sept. 11, 2001, after an airplane crashed into the U.S. military headquarters.

— Robert Turtil / Department of Veterans Affairs via AFP - Getty Images

Firefighters battle flames at the Pentagon on Sept. 11, 2001.

— Hyungwon Kang / Reuters

A person jumps from the north tower of New York's World Trade Center as another clings to the outside while smoke and fire billow from the building.

— Richard Drew / AP

Two women hold each other as they watch the World Trade Center burn.

— Ernesto Mora / AP

Smoke and debris fill the air in New York City as the south tower of the World Trade Center collapses at 9:59 a.m. \"Clearly, not even the police and FBI who had flooded the area were worried about collapse,\" said George Hackett of Newsweek. \"They wouldn't have been anywhere near to the buildings as they were. If the first building hadn't essentially fallen straight down, its crash could have killed hundreds standing, like me, a few blocks away.\"

Smoke and debris fill the air as the south tower of the World Trade Center collapses at 9:59 a.m. "Clearly, not even the police and FBI who had flooded the area were worried about collapse," said George Hackett of Newsweek. "They wouldn't have been anywhere near to the buildings as they were. If the first building hadn't essentially fallen straight down, its crash could have killed hundreds standing, like me, a few blocks away."

— Amy Sancetta / AP

Pedestrians run for cover after the collapse of the south tower.

— Suzanne Plunkett / AP

A woman covered in dust takes refuge in an office building after one of the towers collapsed.

— Stan Honda / AFP

A man stands in the rubble and calls out, asking if anyone needs help, after the collapse of the first World Trade Center Tower.

— Doug Kanter / AFP

A New York City firefighter walks away from ground zero after the collapse of the World Trade Center towers on Sept. 11, 2001 in New York.

A New York City firefighter walks away from ground zero after the collapse of the towers.

— Anthony Correia / Getty Images

A woman on the Brooklyn Heights Promenade reacts to the collapse of the towers.

— Kathy Willens / AP

FBI investigators comb the crater left by the previous day's crash of United Airlines Flight 93 in Shanksville, Pa., on Sept. 12, 2001. The hijacked plane crashed about 80 miles southeast of Pittsburgh after first flying near Cleveland and then turning around.

— Gene J. Puskar / AP

People cross the Brooklyn Bridge to leave lower Manhattan after the collapse of the towers.

— Daniel Shanken / AP

People look back at the fire where the World Trade Center buildings collapsed.

— Mario Tama / Getty Images

RESCUE WORKER REMOVE A MAN AFTER WORLD TRADE CENTER HIT BY PLANE

Rescue workers move the body of the Rev. Mychal Judge, 68, away from the base of the north tower. Judge, the New York Fire Department chaplain, died in the line of duty when struck by debris from the south tower while administering last rites to a firefighter. At 10:28 a.m. the trade center's north tower collapsed and debris flattened the adjacent 5 World Trade Center building and buried the NYFD command post on West Street, killing many senior officials.

— Shannon Stapleton / Reuters

Firefighters raise a U.S. flag where the World Trade Center formerly stood on Sept. 11.

— Thomas E. Franklin / The Record via USA Today Network

New Yorkers stand silently on board the first Staten Island ferry to approach Manhattan after the World Trade Center attack.

— Tom Stoddart / Getty Images

A makeshift altar, constructed for a worship service, overlooks the crash site of United Airlines Flight 93, on Sept. 16, 2001 in Shanksville, Pa.

Photos: Memorials for those lost on 9/11

More NBC News coverage: Remembering 9/11

— Gene J. Puskar / AP

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