The United States marked the sixth anniversary of the September 11th, 2001 terrorist attacks with ceremonies and tributes across the country. Families of the dead, visitors and officials gathered in New York City to mourn victims of the attacks on the World Trade Center. A bell sounded at 8:46 a.m. Eastern time to mark the instant the first hijacked airplane hit the twin towers six years ago. At the White House, President Bush paused for a moment of silence. In New York, Mayor Michael Bloomberg addressed the crowd. "On that day, we felt isolated, but not for long, and not from each other. New Yorkers rushed to the site, not knowing which place was safe or if there was more danger ahead. They weren't sure of anything except that they had to be here. Six years have passed, and our place is still by your side." New York rescue workers who responded to the attacks read the names of more than 2,750 killed that day. This year's anniversary services were moved from Ground Zero because of construction at the site of the former World Trade Center. In a Pennsylvania field, mourners remembered the 40 passengers and crew members killed when one of the hijacked planes crashed. And at the Pentagon, where 184 people died in the crash of a fourth hijacked plane, Defense Secretary Robert Gates vowed to strike back against terrorists. The key planner of the attacks, al-Qaida leader Osama bin Laden, released a videotape praising one of the 19 suicide hijackers. This anniversary was the first that fell on a Tuesday, the same day hijackers crashed planes into buildings six years ago. In all, the September 11th attacks claimed the lives of almost 3,000 people.