Read the complete story here.  But the bottom line is:

A new book written by a former FBI consultant claims that al-Qaida not only has obtained nuclear devices, but also likely has them in the U.S. and will detonate them in the near future.

...

Williams claims that al-Qaida has been planning a spectacular nuclear attack using six or seven suitcase nuclear bombs that would be detonated simulantaneously in U.S. cities.

Ground zero would likely be Washington D.C. and the major population centers.

Sleep well.