NEW YORK (AP) -- After a decade of war with al Qaeda the potential for another devastating terrorist assault "remains very real," Defense Secretary Leon Panetta said Tuesday following a somber visit to ground zero of the Sept. 11 attacks on the World Trade Center.
The Pentagon chief walked through the National September 11 Memorial park and museum with Mayor Michael Bloomberg, and afterward told reporters that Americans must remain vigilant against the threat of another successful al Qaeda attack.
"The potential for that kind of attack remains very real," he said from the 10th floor of 7 World Trade Center, the first completed office tower at the site where hijacked commercial airliners were flown into the Twin Towers, killing more than 2,700 people.
Noting that many of al Qaeda's top leaders, including Osama bin Laden, have been captured or killed in recent months, Panetta said the nature of the terrorist threat has evolved to the point where al Qaeda "nodes" outside of Afghanistan and Pakistan are now the most dangerous.
"Yemen has risen to the top of the list," he said.
Panetta's New York visit gave him an early glimpse at the memorial, with its majestic 30-foot manmade walls of water that hug the sides of one-acre reflecting pools that encompass the ground where the Twin Towers stood. The water is meant to signify falling tears.
Panetta also took a peek inside the entrance to the museum, which is still under construction. And he was shown the "Survivor Tree," a non-fruit bearing pear tree that was rescued from the World Trade Center grounds after the terrorist attacks. It stands amid the white oaks, near the reflecting pool at the former South Tower.
Dozens of workers scrambled Tuesday morning to put the finishing touches on the memorial, planting ivy as ground cover around the 225 white oak trees that surround the reflecting pools.
The Sept. 11 attacks killed nearly 3,000 people, including 184 at the Pentagon. The names of every person who died in the attacks, including those at the Pentagon and in Shanksville, are inscribed in bronze panels that edge the reflecting pools at the Twin Towers site.