Just days before the 10th anniversary of the 9/11 terror attacks, the nation is on high alert after security officials issued a terror warning involving a possible plot by al Qaeda.

Security is being tightened in New York City and Washington, D.C.

In The Big Apple, police on alert, adding extra hours to their shifts.

Officials were increasing the number of on-duty police officers in the nation's capital as well.

"It's accurate that there is specific credible, but unconfirmed threat information," said Janice Fedarcyk with the FBI's New York office.

U.S. officials say al Qaeda's new leader Ayman al-Zawahiri is behind the plot. He has vowed to avenge the death of his predecessor Osama bin Laden, who was killed by U.S. Navy SEALs in May.

Senior federal officials say the plot involves three people, including at least one American citizen who flew into the U.S. from the Middle East in August.

The apparent targets are the same two cities attacked 10 years ago by the terror group, New York City and Washington, D.C.

Officials say this latest plot could involve a car or truck bomb, similar to the failed attack on NYC's Times Square last year.

At least one official said they were searching for two missing rental trucks.

Another possibility is that the terrorists could use small arms, homemade explosives, or even poison.

"Al Qaeda central in Afghanistan and Pakistan is probably down to a few hundred people, but it doesn't take many people to do this kind of attack," explained Richard Clarke, former White House counter-terrorism advisor.

Law enforcement officials want the public to be on the lookout and to not hesitate to contact authorities.

"If you are in a position to see something suspicious, you shouldn't wait. Pick up the phone and call 9-1-1," said James McJunkin with the FBI's Washington office.

While officials say this threat is credible, it's not a cause for panic.

The extra security precautions include increased security at national landmarks, vehicle checks, and bag inspections in subways.