Thirteen years after terrorists used four hijacked airliners to launch the single-most horrific act ever committed on American soil, friends, relatives and even strangers once again made their way to Ground Zero, where Thursday morning loved ones read the names of those lost -- a grim reminder of the toll of Sept. 11, 2001.

The morning after President Barack Obama addressed the nation, vowing to reinvigorate an allied fight against terrorism worldwide, a ceremonial silver bell struck at 8:46 a.m., commencing a moment of silence to honor the time the first plane struck the World Trade Center -- changing America forever in the process.

It was a moment observed by the president and first lady on the lawn of the White House; by those gathered in the plaza at Ground Zero.

Then the reading of the names began, familiar and haunting, as string music played softly in the background. During the reading, loved ones shared remembrances of those lost.

Most in the crowd wore light blue ribbons pinned to their chests, signifying their status as one mourning the loss of a relative.

At one point during the reading, someone let go of a heart-shaped balloon, which reached past the construction cranes, silent for the day, past the skyscrapers, and soon disappeared into the clouds.

It was a day that dawned dark-clouded over New York City and Long Island, as the usual workday groups -- laborers heading to construction jobs, office workers and joggers -- filled lower Manhattan, the place where the Towers were felled on what began as a sun-filled and otherwise ordinary morning 13 years ago.

People in the crowd wore matching T-shirts, cradled bouquets of flowers and held up framed photographs that captured lost loved ones. Some carried American flags.

Others toted small bags of food -- a necessity for the long ceremony at the National September 11 Memorial Plaza. The memorial itself opened in May.

This year is the first that the National September 11 Memorial & Museum is running the commemorative program.

That program includes the reading of the names of all 2,983 killed in the 2001 attacks and the six killed in the 1993 World Trade Center bombing.

Politicians -- including Gov. Andrew M. Cuomo, Mayor Bill de Blasio, New Jersey Gov. Chris Christie, former New York City Mayors Michael Bloomberg and Rudy Giuliani, former New York Gov. George Pataki, Secretary of Homeland Security Jeh Johnson and Sen. Kirsten Gillibrand (D-N.Y.) -- are in attendance. No speeches were planned.

Simultaneous events are taking place nationwide, including in Washington, D.C., where one of the hijacked jetliners was crashed into the Pentagon, and in Shanksville, Pennsylvania, the crash site of United Airlines Flight 93.

The annual "Tribute in Light" -- two columns of light projected skyward where the Twin Towers of the World Trade Center once stood -- will take place at sundown, south of the Ground Zero site. And September 11 museum spokesman Anthony Guido said the plaza will be open to the public for the first time on Sept. 11, allowing visitors "to have a meaningful vantage point of the Tribute in Light."

The public will be allowed to visit the plaza from 6 p.m. until midnight. The plaza and museum will be closed during the day for the ceremonies.

Six moments of silence punctuated the reading of names. The first occurred at 8:46 a.m., the time the first plane struck the north tower.

The second came at 9:03 a.m., when the second plane struck the south tower; the third at 9:37 a.m., the time when American Airlines Flight 77 struck the Pentagon.

The fourth came at 9:59 a.m., when the south tower fell, while the fifth occurred at 10:03 a.m. -- when Flight 93 crashed in Pennsylvania. The last, at 10:28 a.m., marked the time that the north tower collapsed.