OKLAHOMA CITY, Oklahoma (AFP) – Homeland Security Secretary Janet Napolitano urged "vigilance" against violent extremists Monday as the United States marked the 15th anniversary of its deadliest domestic terror attack.

The April 19, 1995, truck bombing of the Alfred P. Murrah federal building in downtown Oklahoma City by members of an anti-government militia killed 168 people, including 19 children, and injured hundreds of others.

"We honor the continued need for vigilance against the hateful ideologies that led to this attack, so that we can recognize their signs in our communities and stand together to defeat them," Napolitano said at a memorial service.

"Above all, we remind ourselves that what defines us as a nation, as a people, and as communities, is not that we've suffered, but how we've risen above it, how we've overcome."

Napolitano cautioned that the United States faces serious threats from both home-grown and foreign terrorists and warned that "our adversaries continue to look for ways to exploit our openness and take innocent lives."

"Terrorism is a tactic designed not just to kill, but to make us feel powerless," she told a crowd gathered at the Oklahoma City National Memorial.

"But we are never powerless. We control the way we prepare ourselves, the way we anticipate and combat the threats, and the way we respond if something does happen."

The federal government is working hard to target its resources against emerging threats and empower the public, she said.

"We can't put a dome over our country. We can't guarantee there won't be another attack. No one can," Napolitano said.

"But we are a strong, resilient country. And we can resolve that even a successful attack will not defeat our way of life."