What is the remaining al Qaeda threat?

The killing of Osama bin Laden was trumpeted on the FBI's list of Most Wanted Terrorists with a single word on a red banner: Deceased.

But, the death of al Qaeda's founder and spiritual leader does not end the terror threat, reports CBS News homeland security correspondent Bob Orr.

Across America Monday, heavily armed police bolstered patrols at airports from Boston to Los Angeles, train stations in New York and San Francisco, and around the U.S. Capitol in Washington.

Special Report: The killing of Osama bin Laden

"We should expect al Qaeda to try to do something," said Rick "Ozzie" Nelson with the Center for Strategic and International Studies. "Sympathizers of bin Laden, sympathizers of al Qaeda in general, will be looking to take action to avenge bin Laden's death."

While sources say there is no credible intelligence pointing to an imminent terror attack, radical Islamist websites are vowing revenge. And in an internal message today to the CIA, Director Leon Panetta issued a stark warning.

"Though Bin Laden is dead, al Qaeda is not. The terrorists almost certainly will attempt to avenge him," he wrote.

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What's next for al Qaeda?

The greatest threat may come from American-born cleric Anwar al-Awlaki and his Yemen-based terror network al Qaeda in the Arabian Peninsula.

Awlaki, and al Qaeda in the Arabian Peninsula have already tried twice to hit the United States. First with the failed Christmas Day underwear bomb plot in 2009, and again last fall with explosives hidden in printers.

And there is increased worry about so-called lone-wolves. Individuals like Maj. Nidal Hasan at Fort Hood, who are moved by al Qaeda's message to strike out in the name of bin Laden and his terror network.

"It may be a mortally wounded tiger that still has some life in it and it's dangerous, and we need to keep up the pressure," said John Brennan, a White House counterterrorism adviser.

The United States has not raised its terror threat level, but U.S. military bases around the world are on alert, and the FBI has now ordered all of its Joint Terrorism Task Forces to press for any evidence of new plots against America.