America mourned the victims of Sept. 11 on Sunday as the siblings of the deceased read their loved ones' names to a weeping crowd at the site of the World Trade Center.

One by one, the names of the dead echoed across the place where the Twin Towers collapsed four years ago in a nightmarish cloud of dust and debris. The ceremony drew to a close after four hours, when all 2,749 names had been read.

Relatives in the crowd bowed their heads and sobbed as speakers uttered brief messages to the brothers and sisters they lost, many of their 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 -- till we meet again."

"My big sister, my better half, life will never be the same without you," Rolando Moreno said to Yvette Moreno, who worked for a brokerage in the North Tower.

As the names were read, mourners filed down a ramp to a memorial reflecting pool at the floor of the site, which remains virtually empty four years after a terrorist attack 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 July terrorist bombings in London. "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.

The Ground Zero ceremony included 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 time when the second plane struck the South Tower; at 9:59 a.m., when the South Tower fell; and at 10:29 a.m., to mark the collapse of the second tower.

Many of the siblings looked to the clear, bright morning sky as they spoke to 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 W. Giuliani and acting New Jersey Gov. Richard J. 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 shot skyward Sunday night to evoke the silhouettes of the towers.

A girl walks across the footprint of the South Tower of the World Trade Center during a ceremony marking the anniversary of the Sept. 11 attacks.