Survivors and the families of those who perished in the 1995 Oklahoma City bombing gathered at the city's National Memorial Monday to commemorate the 15th anniversary of the attack.

Hundreds joined them at the ceremony, which started around the time the bombing occurred, to remember the 168 people killed in the explosion at the Alfred P. Murrah Federal Building on April 19, 1995. More than 600 others were injured.

"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," Oklahoma City Mayor Mick Cornett said.

Department of Homeland Security Secretary Janet Napolitano told those present that the city's spirit in wake of such a tragedy served as an example to the rest of the nation.

"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 overcome," she said. "We can resolve that the Oklahoma Standard becomes the national standard."

Army veteran Timothy McVeigh was convicted of the murders in 1997 and executed in 2001.

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