Fifteen years have passed since the devastating attacks of September 11, 2001. And while most of us remember with unsettling clarity where we were when we heard that hijacked planes had crashed into the World Trade Center (and later, the Pentagon and a Pennsylvania field), killing nearly 3,000 people, it might be the next day — September 12, 2001 — that actually marked the beginning of a new era, one in which full-body scans at the airport, color-coded threat levels, slow-burn wars that never really end, and an undercurrent of fear running beneath the mundanity of life became the norm.

Smoke from the remains of New York's World Trade Center shrouds Manhattan on September 12, 2001. | (REUTERS/Ray Stubblebine)

But, of course, we didn't know that then. On September 12, many Americans felt trapped in a confused and shocking state of limbo. School was canceled. Few people went to work. We were stuck. Some literally, as planes were grounded, subways and trains running minimally if not canceled altogether, and many streets clogged with vehicles. Others emotionally, trapped between grief, shock, fear, and anger.

Below, a look back at September 12, 2001 — the day after the tragedy.

A rescue worker tries to remove dust from an American flag on the debris-covered cemetery of St. Paul's Chapel — across from the World Trade Center — in the early morning of September 12. | (Harald Franzen/ZUMAPRESS.com)

Exhausted firefighters rest in the rubble of the World Trade Center near Church Street in the early morning. | (Harald Franzen/ZUMAPRESS.com)

Residents of downtown Manhattan stop to look at the remains of a car that was destroyed in the attack. The car had been removed from the location where it was destroyed and left in a lower Manhattan side street. | (REUTERS/Brad Rickerby)

People cheer as police officers drive past them in Manhattan. | (Joe Raedle/Getty Images)

People stop and look toward the World Trade Center site from Canal Street in Manhattan. | (Mario Tama/Getty Images)

Crowds wait outside Penn Station in midtown Manhattan, hoping to catch a train out of the city. Penn Station was closed right after the attacks. | (ZUMA Press, Inc. / Alamy Stock Photo)

An interdenominational prayer service at Faithful Central Bible Church in Los Angeles. (David McNew/Getty Images)

Scott LoBaido, right, paints the World Trade Center and American flag onto the front of the Cargo Cafe in Staten Island, New York, with the help of co-owner Dave Rabin. | (DOUG KANTER/AFP/Getty Images)

U.S. National Guard troops march down West Street near the wreckage of the World Trade Center. | (REUTERS/Mike Segar)

Federal investigators search for clues near the damaged area of the Pentagon building as firefighters and rescue workers continue to battle smoke at the U.S. military headquarters outside of Washington. | (REUTERS/Larry Downing)

President George W. Bush speaks to reporters after viewing the devastation at the Pentagon. Accompanied by Secretary of Defense Donald Rumsfeld, Bush thanked rescue workers and volunteers for their efforts as he stood in front of the damage. | (AP Photo/Pablo Martinez Monsivais)

Military and fire personnel unfurl a large American flag on the roof of the Pentagon. | (AP Photo/Steve Helber)

A U.S. Park Police officer maintains watch in front of the flags flying at half mast at the Washington Monument. | (ROBYN BECK/AFP/Getty Images)

Photographs of missing people from the World Trade Center attack cover a television truck outside Bellevue Hospital in New York, where family members and friends stand vigil in hopes of getting information from authorities and help from media exposure. | (JOHN MOTTERN/AFP/Getty Images)

A candlelight vigil in Las Vegas. | (REUTERS/Ethan Miller)

Thousands of people take part in a candlelight vigil on the mall in Washington, D.C. | (REUTERS/Win McNamee)