One by one, the names of the dead echoed across the site where the twin towers collapsed four years ago in a nightmarish cloud of dust and debris.
Relatives in the crowd bowed their heads and sobbed as speakers uttered brief, personal messages to the brothers and sisters they lost, many voices breaking in sorrow.
"Mom and Dad ache for you every minute," Linda Giammona-Julian said to her brother, Vincent Giammona, one of 343 firefighters killed. "We love you and we miss you; til we meet again."
As the names were read, weeping mourners filed down a ramp to a reflecting memorial pool at the floor of the site, which remains virtually empty four years after the attack killed 2,749 people and tore a hole in the New York skyline. Families filled the water with red, orange and yellow roses, some shaking as they inscribed dedications on the wooden edge of the pool.
The ceremony came as Hurricane Katrina left Americans once again struggling with a catastrophe that caught the nation unprepared and left citizens dead and grieving.
Mayor Michael Bloomberg opened with words of condolence for those devastated by Katrina and the terrorist bombings in the London Underground.
"To Americans suffering in the aftermath of Hurricane Katrina, our deepest sympathies go out to you this day," Bloomberg said.
In New Orleans, New York firefighters helping with the relief effort gathered around a makeshift memorial for their fallen comrades, accepting the gift of a bell from a nearby church whose steeple was destroyed in the storm.
The ground zero ceremony paused for moments of silence at 8:46 a.m., the time at which a hijacked jetliner crashed into the north tower, at 9:03 a.m., the moment a second plane struck the south tower, at 9:59 a.m., when the south tower fell and at 10:29 a.m., when the second tower collapsed.
Many relatives looked to the clear, bright morning sky as they spoke to the brothers and sisters they lost. Several held up photos of their loved ones.
"You're taking care of us from heaven but someday we'll be together," Iliani Flores said, choking up and raising her face to the sky in memory of her younger brother, a fire department paramedic.
Secretary of State Condoleezza Rice read a poem by Christina Rossetti after the second moment of silence. Gov. George E. Pataki, former Mayor Rudolph Giuliani and New Jersey Gov. Richard Codey also addressed the crowd.
"We all stand together to help each other and to help those who need our help in the future," Giuliani said. "We remember forever all the brothers and sisters that we lost on that day."
In Washington, President Bush marked the anniversary with his wife on the South Lawn, and throngs of people marched in remembrance of the attacks and in tribute to troops fighting overseas.
And in southwestern Pennsylvania, about 1,000 people attended a memorial service in the field where Flight 93 crashed after it was hijacked by terrorists.
Other memorials planned Sunday included a Port Authority of New York and New Jersey service for the 84 employees it lost on Sept. 11. Firefighters planned to roll out their trucks and other equipment in front of their firehouses.
Parents and grandparents read the victims' names at ground zero last year, while children's voices were heard in 2003. A selection of politicians, relatives and others read the names on the first anniversary.
Two light beams inspired by the twin towers were to shoot skyward Sunday night in an echo of the towers' silhouette. The "Tribute in Light" will fade away at dawn on Monday.