Published: Apr 19, 2010 8:25 PM EDT

OKLAHOMA CITY, Oklahoma - U.S. Homeland Security Secretary Janet Napolitano told survivors and victims' relatives gathered Monday, April 19, 2010, for a somber ceremony to mark the 15th anniversary of the Oklahoma City bombing that the city's spirit in the wake of the tragedy served as an example to the nation.

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."

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.

After the ceremony, family members and survivors gathered again on the building's footprint. Nearby an American Elm, known as the "Survivor Tree" because it survived the blast, bloomed a brilliant shade of green. Most of those in attendance sat quietly in a grassy area near the Survivor Tree and a reflecting pool, politely applauding the speakers, including Gov. Brad Henry and former Gov. Frank Keating, who was in office when the blast occurred.

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.

Attending the ceremony was 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.

Hangar, now the Noble County sheriff, read the memorial's mission statement at the start of the service. U.S. Rep. Mary Fallin, R-Okla., who was the state's lieutenant governor at the time of the bombing, read a congressional resolution commemorating the anniversary. Napoilitano 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."

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 website.

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.