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Michael Leiter, acting director of the National Counterterrorism Center, told a Senate hearing last week that al Qaeda in particular is continuing to seek unconventional weapons.

“Most troubling is the judgment they will continue to try to acquire and use chemical, biological, radiological or nuclear materials in attacks,” Mr. Leiter said.

Other senior U.S. intelligence officials have said that one threat scenario is for terrorists to set off a radiological bomb — a conventional bomb laced with nuclear material to enhance its lethality.

Stealing or constructing a nuclear bomb for use in an attack would be more difficult but not impossible.

“More than anywhere else in the nuclear weapons complex, it is essential to prevent terrorists from accessing the nuclear materials at Livermore,” said Danielle Brian, POGO’s executive director. “Suicidal terrorists would not need to steal the materials; they simply could detonate them into an improvised nuclear device on the spot. That is why it is urgent to remove those materials from the lab, rather than settling for the [Energy Department’s] drawn-out timetable of removing the materials by 2012. We hope this debacle will finally light a fire under [the Energy Department] and accelerate their schedule.”