At a briefing kicking off President Obama's nuclear security summit on April 12, presidential adviser John Brennan said al Qaeda in particular has been actively trying to acquire a nuclear weapon for the past 15 years.

"Developing a nuclear device involves a highly sophisticated technical process, and al Qaeda doesn't seem to have mastered it based on what we know now."

Organized crime and criminal gangs are well aware of the terrorist group's interest in acquiring bomb-making materials, which has led criminals to pursue getting those items for their own profit, according to Brennan.

Nuclear weapons expert David Albright said it is unclear what terrorists are up to and when they might act.

Albright was worried about whether Pakistan's nuclear stockpile is secure, despite assurances from both the U.S. and Pakistan that it is.

Pakistan "has had many leaks from its program of classified information and sensitive nuclear equipment, and so you have to worry that it could be acquired in Pakistan," said the author of "Peddling Peril."