Ceremony Marks 15th Anniversary of Oklahoma City Bombing

Homeland Security Secretary Janet Napolitano joined survivors and victims' family and friends for a ceremony today at the site of the Oklahoma City bombing, where 168 people lost their lives in a terrorist attack 15 years ago today. Napolitano and other dignitaries gathered alongside the reflecting pool of the Oklahoma City National Memorial, which now stands where the Alfred P. Murrah Federal Building did before it was destroyed by Timothy McVeigh on April 19, 1995.

VIDEO: Oklahoma City remembers the terrorist bombing of the Murrah Federal Building.Play

Napolitano, who helped lead the federal government's investigation into the attack as U.S. attorney in Arizona, honored the victims and told of the important lessons learned from the bombing. She praised the strong spirit of Oklahoma and the nation in the wake of the attack, which was the deadliest terrorist attack on American soil at the time.

"We cannot put a glass dome over our country. We cannot guarantee there will not be another attack. No one can," said Napolitano. "But we are a strong and resilient country. And we can resolve that even a successful attack will not defeat our way of life."

Standing between the memorial's two bronze gates marking the minutes before and after the attack, Napolitano recognized the importance of remembrance without being fearful.

VIDEO: Oklahoma City Bombing 1995Play
April 19, 1995: Oklahoma City Bombing

"Terrorism is a tactic designed not just to kill but to make us feel powerless," she said. "But we are never powerless. We control the way we prepare ourselves, the way we combat threats, and the way we respond if something indeed happens."

The Oklahoma Standard

Former Oklahoma Gov. Frank Keating, who was in office during the attack, took time to praise the the heroism and professionalism of the first responders who helped pull hundreds of victims from the rubble. He also praised the outpouring of support for those first responders from the Oklahoma community, which has come to be known as the Oklahoma Standard.

"Anytime anyone asked for anything, it was given them," he said. "Many of the rescue workers who are here today remember -- never ask for batteries, cause they bring you truck loads. Just ask for 3 or 4 -- whatever you need."

VIDEO: Timothy McVeigh ExecutedPlay
June 11, 2001: Timothy McVeigh Executed

168 Seconds of Silence

Before the ceremony, family and friends of the victims placed wreathes, bouquets of flowers, and teddy bears on the memorial's 168 empty chairs. At 9:02 a.m., the exact moment of the bombing, people bowed their heads and paused for 168 seconds of silence.

School children across the state of Oklahoma also marked the anniversary with a moment of silence. Memorial officials and state leaders addressed the importance of teaching the history and lessons of the bombing to a generation of children who were not even born when the attack happened. Oklahoma Gov. Brad Henry recently signed a law requiring the history of the attack be a part of the state's permanent school curriculum.

McVeigh was executed in 2001 after being convicted on federal murder charges. His Army friend, Terry Nichols, is serving a life sentence in a Colorado federal prison after being convicted on state and federal bombing charges.

15th Anniversary of Oklahoma City Bombing: Reading of Victims' Names

In the most emotional part of the ceremony, the names of the 168 victims were read aloud, grouped according to the floor they were on during the bombing. With their voices sometimes quavering, sons, daughters, brothers, sisters, parents, and spouses of the victims took their turn at the microphone. The victims came from different parts of the federal government -- from the U.S. Secret Service to the Social Security Administration to the America's Kids day care center, where over a dozen young children lost their lives.

One of those children, Baylee Almon, would have turned 16-years-old this week. In the years since the attack, her mother has turned her grief over the loss into action, crusading to make federal buildings safer with shatter-resistant glass.

"Her death, to me, wasn't for nothing," said Aren Almon-Kok. "It was so maybe others could make a difference. So other people can go home to their families at night."

Children Who Survived the Attack Now in College

Many of the children who survived the explosion are now in college and high school. Chris Nguyen was in the day care center, and doctors weren't sure the 5-year-old would live. Today, he is a sophomore at Oklahoma University.

"After all these years of researching it and figuring out what happened, kind of piecing everything together myself, it stuns me that the survivors made it through," Nguyen said.

The ceremony ended with bagpipe music, and state leaders promised to remember the lessons of the bombing and the way it brought a community together.

"The Oklahoma Standard is not a past event," said Gov. Brad Henry. "It is part of the character and fabric of the people of this city and state."

ABC's Ryan Owens contributed to this report from Oklahoma City.