Brian Williams; Virginia Tech Massacre; Richard Perle

NBC News anchor Brian Williams shares his interview with President Bush about the Virginia Tech massacre, and reports on what is currently known about the shooter Seung-Hui Cho's motives. Correspondents Brian Ross and Michael Shear share new information on Virginia Tech shooter Seung-Hui Cho. Richard Perle, former assistant secretary of defense under Reagan and chairman of the Defense Policy Board under George W. Bush, shares his view of U.S. defense policy, with a look at his appearance in a new PBS series, "America at a Crossroads."