Two top U.S. senators are sounding the alert that the U.S. could be exposed to imminent retaliatory attack from a lone-wolf terrorist moved to action with the death of al Qaeda Leader Usama bin Laden, though there is no specific or credible information that any attack is imminent.

"My own great concern in the days ahead is a so-called 'lone wolf', a single individual acting alone, who will now mobilize himself or herself," warned Senate Homeland Security Committee Chairman Joe Lieberman, I-Vt. "My appeal to the American people is be alert. This is a classic 'If you see something, say something' alert," the senator said, referring to the Department of Homeland Security's public awareness campaign to get ordinary Americans involved in pointing out suspicious activity and persons.

Lieberman's GOP committee counterpart, Susan Collins of Maine, echoed that same concern "that a home-grown terrorist will attempt to retaliate," but the senator said she was "particularly pleased" that homeland security officials on Sunday night issued a so-called "situational awareness alert" to all homeland security officials across the country "giving them some intelligence" and warning them to remain alert.

In general, the two senior lawmakers praised the extraordinary operation for its cross-government coordination, something that both said did not exist before the September 11, 2001 terrorist attacks.

The chairman said there "may now be heightened level of danger as individuals seek revenge," but he added that Americans should be assured that "all (government) agencies are alert to those dangers."

When asked why DHS had not raised the country's terror alert, Lieberman, who said he spoke with the department's secretary, Janet Napolitano, Monday morning, said, "I think her feeling is that under the new system that in order to raise the alert level here at home to either ‘elevated' or ‘imminent', but certainly ‘elevated', there needs to be specific and credible evidence of a plot against the United States here at home. And at this point, there is no such evidence. They are on what I would describe as a kind of unofficially high alert, because for the natural reason that they worry that there's going to be a retaliatory attack. But thank God so far they have no specific and credible evidence, and if they raise the threat level to ‘elevated', you'll know that they have it."