05/02/2011 01:36 pm ET | Updated Jul 02, 2011

'Lone Wolf' Revenge Attack Possible: Joe Lieberman

WASHINGTON -- Americans should be on the alert for possible lone-wolf action to avenge Osama bin Laden’s death, Sen. Joe Lieberman (I-Conn.) said on Monday.

The Department of Homeland Security has not increased the threat level, but Lieberman told reporters he believed the agency is operating under an “unofficial high alert” following the Sunday killing of the al Qaeda leader. Department of Homeland Security head Janet Napolitano said in a Monday statement that DHS would not raise the National Terror Advisory System threat level unless there was a “specific and credible” threat of violence.

But Lieberman said at a press conference that he fears a lone actor, perhaps a home-grown terrorist “who has become radicalized,” will attempt an attack within the United States in the coming days or weeks.

“These are the most difficult cases to see and stop,” Lieberman said. “My appeal to the American people is please, be alert. This is a classic ‘If you see something, say something’ moment.”

Sen. Susan Collins (R-Maine) said at a press conference that DHS had provided information to state and local law enforcement to help them detect potential threats.

To help deter potential terrorist attacks coming from outside the country's borders, both Collins and Lieberman said it may be necessary for the Obama administration to release photographs of the al Qaeda leader's body. Bin Laden was reportedly shot in the head, and the Obama administration has not released the photos, due to their gruesome nature. But Lieberman said al Qaeda members may need to see the photos to believe the United States killed bin Laden.

“It may be necessary to release the pictures ... to quell any doubts that somehow this is a ruse that the United States government has carried out,” he said. “My own instinct is that it’s probably necessary to release those pictures, but I will respect whatever decision the president makes.”