() A former CIA officer who helped fight the war on terror after Sept. 11 says another attack against the U.S. remains possible.

national security contributor Tracy Walder was part of the effort to track down 9/11 mastermind Osama bin Laden, who was killed by a Navy SEAL team in 2011. She tells “The Hill on ” that taking out terrorist leaders doesn’t eliminate the broader threat of organizations with extreme beliefs.

“You can kill all of these heads, but you cannot kill the ideas that come with it,” Walder said Thursday on the 24th anniversary of the Sept. 11 attacks that claimed the lives of nearly 3,000 Americans. “These ideas are more profuse than ever, particularly with rampant social media.”

Nearly a quarter-century after al-Qaida struck, Walder expressed regret over U.S. intelligence failures that missed the 9/11 plot.

“The fact of the matter is, we did miss it. Obviously, we didn’t miss it on purpose. Because of that, thousands of innocent people are dead,” she said.