Terror threat clouds start of 9/11 ceremonies

US officials urged Americans to be on the alert as the nation began marking the poignant 10th anniversary of the 9/11 attacks amid warnings of a new Al-Qaeda linked terror threat. Although details of the new suspected plot possibly involving car bombs were sketchy, a US official told AFP the threat was credible and somewhere between "aspirational" and a "boom." US Secretary of State Hillary Clinton acknowledged during a speech at the New York Stock Exchange that it had been decided to reveal the new threat just days ahead of Sunday's anniversary in a bid to thwart any plot. "Making it public... is intended to enlist the millions and millions of New Yorkers and Americans to be the eyes and the ears of vigilance," she said, after a minute's silence was held on the exchange floor to honor the 9/11 dead. It would seem from the threat report "that Al-Qaeda, again, is seeking to harm Americans and, in particular, to target New York and Washington," Clinton added. Homeland Security Secretary Janet Napolitano also called on all Americans to remain vigilant and report anything suspicious, vowing to "protect the American people from an evolving threat picture both in the coming days and beyond." Somber ceremonies began around the country Friday to mark the 10th anniversary of the September 11, 2001 attacks as armed police patrolled New York streets and subways. Memories remain raw of the day when Al-Qaeda hijackers slammed three passenger planes into the World Trade Center, destroying its iconic Twin Towers, and the Pentagon, in the nation's capital. A fourth plane crashed in a Pennsylvania field when the passengers valiantly overpowered the hijackers. Almost 3,000 people were killed that day in the worst ever attacks on American soil. Former national security advisor Frances Townsend told CNN Friday that US spy networks had been alerted to a new threat after intercepting communications from a known, reliable operative in Pakistan. "It's Washington or New York. A car bomb, three men. We know that one or two are US citizens," she said, when asked about the specifics of the threat. "We have not heard yet whether or not -- do we have identifiers on these people, do we know their locations? Presumably not," she said, adding there was still a lot of work needed "to be able to put this to bed or disrupt the plot." "The general outlines of the initial report are three individuals coming into the country" last month, a US official told AFP, confirming the plot had links to militants in Pakistan. President Barack Obama was briefed again Friday on the threat and repeated his order for security agencies to "redouble" efforts to take all necessary precautions, his spokesman Jay Carney said. But there have been no changes to his plans to attend Sunday's ceremonies on at Ground Zero in New York, and Shanksville, Pennsylvania. A memorial to the victims of United Airlines Flight 93 who died in that Pennsylvania field will be unveiled in a solemn ceremony on Saturday. In New York, heavily armed police patrolled the busy streets, trucks and cars were stopped and inspected at vehicle checkpoints and bomb-sniffing dogs scoured the subway. "I would say that people should be alert. I don't think anybody should be panicked," Senator Joe Lieberman, chairman of the Senate Homeland Security Committee, told CNN. The decision to go public with the information was "important because it alerts everybody to be on guard this weekend. And be careful," he added. ABC television said the latest plot was ordered by Al-Qaeda's new leader Ayman al-Zawahiri, who has vowed to avenge the death of Osama bin Laden in a US raid on his Pakistani compound in May. Former vice president Dick Cheney meanwhile defended the previous administration's "war on terror" launched after the attacks, and the use of tough interrogation techniques -- denounced as torture by rights groups. "Three people were waterboarded, not dozens, not hundreds, three," he said, insisting the interrogations provided valuable counter-terrorism information and had made the nation safer. America's response to 9/11 showed that when attacked, "we will come and get you," Defense Secretary Leon Panetta warned Friday as he thanked the firefighters and emergency teams who rushed to the Pentagon after it was struck. "The people who attacked us on 9/11 were trying to weaken America, trying to hurt America. And instead they strengthened us," Panetta said. "Because you don't mess with this country."  Former president George W. Bush will lay a wreath at the Pentagon in a private ceremony on Saturday and is also due to attend Sunday's ceremonies.