(Sue Ogrocki ~ Associated Press)
Napolitano also warned of the need for continued vigilance against terrorists when she spoke during the 90-minute memorial to the 168 lives lost in the destruction of the Alfred P. Murrah Federal Building on April 19, 1995. More than 600 others were injured in the blast, which at the time was the deadliest terrorist attack on U.S. soil.
Across Oklahoma City, people observed 168 seconds of silence to honor the dead.
Some dabbed away tears as the ceremony closed with family members reading a roll call of those who died.
"What defines us as a nation, as a people and as communities is not what we have suffered, but how we have risen above it, how we've overcome," Napolitano said.
"We can resolve that the Oklahoma Standard becomes the national standard," she said of the willingness of Oklahomans to help those in need without compensation.
The ceremony followed a time-honored script. Shortly before 9:02 a.m. -- when the bombing occurred -- bells tolled in downtown Oklahoma City. Some family members visited the site of the federal building razed in the attack and left ribbons, wreaths and other objects on chairs that stand on the site to honor the dead.
Vickie Lykins and her sister, Angela Richerson, placed a rose, an American flag and a purple ribbon on the chair honoring their mother, Norma "Jean" Johnson, who had been a Defense Security Service worker.
"This is our mother's favorite color," Lykins said as she secured the ribbon.
"Time heals nothing," said Debi Burkett Moore, who placed a floral display on the seat and back of the chair honoring her brother, David Burkett, a U.S. Department of Housing and Urban Development. She said the ceremony "makes it a little more bearable, but it heals nothing."
Kathryn Burkett, David Burkett's mother, said she is saddened more by her son's death with each passing year.
"Why it is sadder, I don't know why," Burkett said. "You just live with it."
After the ceremony, family members and survivors gathered again at the building's footprint. Nearby an American Elm, known as the "Survivor Tree" because it survived the blast, bloomed a brilliant shade of green.
Oklahoma City Mayor Mick Cornett said the city remembered the day of the bombing with reverence, "not because we can't forget but because we choose to remember.
"We have chosen strength, we have chosen optimism, we have chosen freedom, we have chosen to move forward together with a level of unity that is unmatched in any American city," Cornett said at the ceremony, held on a cool, overcast morning.
Gov. Brad Henry said legislation he signed earlier this month would ensure that students learn about the bombing and its aftermath in history classes.
"We have a duty to assure that future generations remember those lost and injured here, that they understand the lessons of this vital part of our shared history," he said.
Charlie Hangar -- the Oklahoma Highway Patrol trooper who stopped bomber Timothy McVeigh on Interstate 35 the day of the blast because his 1977 Mercury Marquis did not have a license plate -- read the memorial's mission statement at the start of the service. Hangar is now the Noble County sheriff.
Napolitano said the bombing anniversary was a reminder of "the continued need for vigilance against the violent ideologies that led to this attack, so that we can recognize their signs in our communities and stand together to defeat them."
"We cannot put a glass dome over our country. We cannot guarantee there will not be another attack. No one can," Napolitano said. "But we are a strong and resilient country. And we can resolve that even a successful attack will not defeat our way of life."
In a documentary, "The McVeigh Tapes: Confessions of an American Terrorist," to be aired Monday on MSNBC, recordings of interviews with the convicted bomber indicate he had no remorse for those whose lives he had destroyed.
"Throughout the history of mankind, people have killed for what they believed was the greater good and ... and it's accepted. Sometimes killing is accepted," McVeigh told journalists Lou Michel and Dan Herbeck in comments posted on the MSNBC Web site.
Prosecutors had said McVeigh's plot was an attempt to avenge the deaths of about 80 people in the government siege at the Branch Davidian compound in Waco, Texas, exactly two years earlier.
McVeigh was convicted on federal murder charges and executed in 2001. McVeigh's Army buddy, Terry Nichols, was convicted on federal and state bombing-related charges and is serving multiple life sentences at a federal prison in Colorado.