This NYT article looks at how the U.S. has focused on learning from the lessons of 9/11 while forgetting about the possibility of new emerging security threats.

An excerpt:

The Department of Homeland Security has taken significant steps since the Sept. 11 terrorist attacks to make it much harder to turn a plane into a flying weapon. But a nearly obsessive focus on the previous attacks may have prevented the federal government from combating new threats effectively, terrorism experts and former agency officials say.

The arrests overseas this week of people accused of planning to use an explosive that would be undetectable at airports illustrates the significant security gaps, they said.

While the department has hardened cockpit doors and set up screening for guns and knives, it has done far too little to protect against plastic and liquid explosives, bombs in air cargo and shoulder-fired missiles, the experts say.

The nation is still at risk from the same “failure of imagination” cited by the 9/11 commission as having contributed to the success of the 2001 attack, several argued.

“They are reactive, not proactive,” said Randall J. Larsen, a retired colonel in the Air Force who is chairman of the military strategy department at the National War College in Washington.

Robert M. Blitzer, who served 26 years in the Federal Bureau of Investigation, including as head of its counterterrorism unit, said the federal government had a serious problem because its personnel today turned over far too quickly.

Mr. Blitzer, now an analyst at ICF International, in Fairfax, Va., said: “They don’t have enough continuity and knowledge to know what they’re up against. Stability is a big thing for identifying trends. It’s not easy to do. Sometimes all you have is just snippets of information.”

Justin P. Oberman, a former senior policy official at the Transportation Security Administration, said the problem was not lack of imagination but limited money available to invest in the technologies needed.

“Too much is weighted toward looking for knives and guns on people coming through the checkpoint and screening every checked bag,” Mr. Oberman, who left the agency last year, said.

Homeland Security Secretary Michael Chertoff, in a news conference Friday, said the department was trying to stay ahead of terrorists.

“We’ve spent about three-quarters of a billion dollars in research on emerging types of technologies in explosives,” Mr. Chertoff said. “And we are constantly monitoring the world for developments that occur in the field of improvised explosive devices, precisely so we can start to work on countermeasures.”

But even at senior levels of the department, there is recognition that this criticism is somewhat fair. “D.H.S. has to be nimble in a way most government agencies don’t, and that has to be baked into our very DNA,” said Michael Jackson, the deputy secretary, in an interview. “I am impatient. I don’t think we have gotten as far as we need to go. We can do more, and we can do better. And we must.”