WASHINGTON, Sept. 9 (UPI) -- Osama bin Laden's successor is believed to be behind a possible plot to attack New York and Washington around the Sept. 11 anniversary, U.S. officials said.
Ayman al-Zawahiri, who last month urged Muslims in a video message to target the United States to avenge bin Laden's killing, initiated the alleged plot, ABC News reported, citing intelligence officials.
Other U.S. officials were less specific, telling The Wall Street Journal al-Qaida leaders in Pakistan were believed to be behind the alleged plot.
Department of Homeland Security spokesman Matt Chandler called the threat "specific, credible, but unconfirmed."
U.S. Vice President Joe Biden said Friday authorities have a "credible" but unconfirmed information on a possible terrorist car bombing in New York or Washington
"We do have talk about using a car bomb. We've been told that was an intention ... from a credible source," Biden said on ABC's "Good Morning America."
"But we do not have confirmation of that. We don't have a smoking gun."
Biden said he and President Barack Obama had been briefed about CIA intelligence that three men may have entered the country intending to use a vehicle bomb in a terrorist attack. But he said there is "no certitude" an attack had been planned.
U.S. intelligence agencies urgently hunted leads overseas to gauge the threat's seriousness, officials said.
"Pursue America, which killed the 'imam of the Mujahedin' and threw his body into the sea, and then captured his women and sons," Zawahiri said on the video, referring to bin Laden, killed May 2 in a covert U.S. raid in Pakistan.
Three current al-Qaida leaders believed to present a particular threat to the United States because they've lived in the country are Adnan el-Shukrijumah, alleged to have been involved in the 2009 New York subway bomb plot; Jude Kenan Mohammad, an American alleged to have helped recruit five Alexandria, Va., men; and Adam Gadhan, an American al-Qaida spokesman, The Wall Street Journal reported.
At least three suspects in the reported plot, including an American citizen, are believed to have entered the United States by air last month after leaving Afghanistan, a counterterrorism official citing a U.S. intelligence report told ABC News and The New York Times.
They are believed to have originated from Pakistan tribal areas along the Afghan border, The Washington Post reported.
Two of the individuals may have had U.S. documentation, either passports or "green cards" indicating they were permanent U.S. residents, ABC News said.
Intelligence agencies have not identified the individuals.
President Barack Obama, who ordered the U.S. Navy SEALs raid that killed bin Laden, was briefed on the possible threat multiple times Thursday and directed U.S. intelligence officials to "take all necessary steps to ensure vigilance," the White House said.
A bin Laden notebook seized after the al-Qaida leader was killed indicated he wanted to attack the United States on or around this year's 10th anniversary of the Sept. 11 terrorist attacks, U.S. officials said.
Americans and officials across the country, including Obama and former President George W. Bush, are to observe the attack anniversary Sunday with high-profile events at each attack site -- New York City, Shanksville, Pa., and the Pentagon in Virginia.
New York Mayor Michael Bloomberg, appearing with Police Commissioner Raymond Kelly and other officials Thursday night, said the New York Police Department would deploy additional resources to keep residents safe. He said New Yorkers should go about their business as usual and the city's 9/11 observance will go ahead as planned.
The area around Ground Zero had already been deemed a "frozen zone" Sunday, with police cordoning off several blocks in all directions, forcing even residents to be escorted by police officers to their apartments.
In Washington, extra uniformed and plainclothes officers, supported by police cruisers and SWAT teams, will be near the U.S. Capitol and other locations, Police Chief Cathy Lanier said. The FBI will boost its presence, with more marked cars and agents earmarked for key spots in the nation's capital.
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