Lieberman calls for heightened vigilance: ‘If you see something, say something’

"If you see suspicious behavior, call the police immediately — and that includes if you see suspicious behavior by someone who is a friend or family member," Lieberman said Monday in a Capitol press conference.

Lieberman said his experience leading the congressional investigation into the Fort Hood shootings have made him especially concerned that a "lone wolf" terrorist could strike domestically in reprisal for the killing of bin Laden.

"My own grave concern in days ahead is that a so-called lone wolf that been radicalized will mobilize himself now to take action here against the American people," said Lieberman.

"These are the most difficult cases to see and stop," he added.

President Obama announced Sunday night that bin Laden had been killed in a firefight in Pakistan by U.S. special forces.

The U.S. government has since issued worldwide security warnings for American citizens living or traveling abroad, and government agencies have ordered their security services to remain alert at home.