It's another ominous sounding threat, but one many on the East Coast have gotten used to in the decade since the attacks.
Officials describe the latest threat as credible but unconfirmed.
The information comes from a source in Pakistan indicating al Qaeda's new leader, Ayman al-Zawahiri, wants to strike the U.S. around the anniversary of Sept. 11.
Authorities say that three suspects flew from Pakistan to the U.S. in August, and at least two of them are believed to be U.S. citizens or have American traveling documents.
Officials told ABC News their apparent targets were the same two cities attacked 10 years ago.
"The new leader, Zawahiri, has said he wants an attack around now to avenge bin Laden and to prove that al Qaeda central, the one in Pakistan and Afghanistan, is still alive," said Richard Clark, ABC security expert.
Officials said the plot could involve a car or truck bomb similar to the failed attack on Times Square last year. Police are now giving vehicles even more attention, and security has been heightened in and around both cities.
Still, local leaders don't want citizens to live in fear.
"I just think that if I take the subway, it sort of puts it in perspective that there's always a threat, but there's not any reason why we should hunker down," said New York Mayor Michael Bloomberg.
So far, Los Angeles has not been mentioned in the intelligence gathered, but the city has been the target of past attacks. Authorities are reminding citizens to stay alert.
"If you see something, as a member of the public, that doesn't make sense to you, particularly on this anniversary of the most terrific attack to occur on American soil, say something," said LAPD Chief Charlie Beck.
There are indications the attack could be aimed at bridges and tunnels. Police in New York and the Washington D.C. area warned residents that cars left unattended near sensitive targets will be towed.
