NEW YORK CITY -- One after another Sunday morning, the names of people who died in the Sept. 11, 2001 terrorist attacks echoed across the site where the World Trade Center towers once stood in downtown Manhattan.

The ceremony marked the fourth anniversary of the attacks.

More than 600 family members who lost brothers or sisters in the attacks gathered at Ground Zero to read the names, while hundreds of others watched in pained silence, some holding portraits of their loved ones.

One sign read, "We love you. We miss you."

Among the mourners was Linda Giammona-Julian, whose brother Vincent was one of 343 firefighters killed that day. She said, "Mom and Dad ache for you every minute."

As the names were read, mourners filed down a ramp to a reflecting memorial pool at the floor of the site. They filled the water with roses -- red, orange and yellow under a clear, blue sky.

There were four moments of silence at the site to note the moments that the planes hit the World Trade Center and the times that the twin towers fell.

Two jetliners, hijacked my Muslim extremists, struck the first tower at 8:46 a.m., and the other at 9:03 a.m. The first tower collapsed at 9:59 a.m., the second at 10:29 a.m.

A third hijacked jet smashed into the Pentagon shortly afterward.

A fourth hijacked jet, Flight 93, crashed into a Pennsylvania field.

Thousands of people in a quiet, somber mood remembered the Pentagon attack as they sang "God Bless America" outside the Pentagon and marched in remembrance of the attacks and in tribute to troops fighting abroad.

The Defense Department organized the "America Supports You Freedom Walk" as a mixed commemoration of the anniversary with a call to honor those serving in the armed forces.

During a wreath-laying ceremony at nearby Arlington National Cemetery, Defense Secretary Donald Rumsfeld told the crowd history is being written in a test of wills.

He noted that children of some of the 184 who died at the Pentagon were in attendance.

"It's hard for free people to comprehend the mix of extremism and hatred that leads terrorists to murder innocent men, women and children," Rumsfeld said

President George W. Bush, first lady Laura Bush, Vice President Dick Cheney and his wife Lynn stood on the South Lawn with their hands over their hearts -- as did other administration officials and their families who joined in the solemn observance.

The commemoration followed a private worship service at St. John's Church, across Lafayette Park from the executive mansion.

About 1,0000 people were on hand at Sunday's memorial service for those who died four years ago on Flight 93 in Pennsylvania.

Bells tolled in the town of Shanksville, near the crash site, as local volunteers read aloud the names of each victim.

One of those in attendance said the victims were the first heroes of the Sept. 11 attacks.

The official 9/11 Commission report said hijackers crashed the plane in southwestern Pennsylvania as passengers tried to take control of the cockpit. The report's authors believe the hijackers had targeted either the White House or the U.S. Capitol.

U.S. Attorney General Alberto Gonzales, Sen. Rick Santorum, R-Pa., Pennsylvania Gov. Ed Rendell and former Homeland Security Secretary Tom Ridge were at the memorial service.

Ridge and Gonzales said those who died saved the lives of others on the ground.

Pope Benedict marked the anniversary with a call for less hatred and more love.

His comments came as he greeted those who gathered Sunday in the courtyard of the papal summer palace south of Rome.

The pope said in English, "May God inspire men and women of good will everywhere to renounce hatred and to build a world of justice, solidarity and peace."

The anniversary comes as eight in 10 people in the United States think the threat from terrorism is about the same or has grown worse than it was at the time of the attacks.

The CBS poll found that about half of Americans think the threat is at the same level and three in 10 say it has grown worse.

Additionally, six in 10 said they have confidence in the government's ability to protect citizens from terrorism, down from seven in 10 who felt that way before Hurricane Katrina hit.

A CNN USA Today-Gallup poll finds people are evenly split on whether they are satisfied with the way things are going in the war on terrorism, with 51 percent satisfied and 49 percent dissatisfied.

Four in 10 say their life changed after Sept. 11 and they do not expect it will return to normal, according to the poll.