President Bush and Cabinet members somberly marked the sixth anniversary of the Sept. 11 attacks with ceremonies at the White House and the spot where terrorists flew a plane into the Pentagon.

Bush, joined by his wife, the vice president, Cabinet members, White House janitors, kitchen workers and groundskeepers observed a moment of silence at the White House at 8:46 a.m., Tuesday — the exact moment in 2001 when terrorists slammed the first of two jetliners into the World Trade Center in New York.

Across the Potomac River in suburban Virginia, top Defense Department officials held a moment of silence at 9:47 a.m., the moment when the third terrorist plane crashed that day, killing 184.

The president stood with first lady Laura Bush and Dick and Lynne Cheney on the South Lawn of the White House during the simple ceremony that has been repeated each year since the attack.

Bush stood sternly as the Marine band, stationed behind him on the South Portico of the White House, played "God Bless America." Mrs. Bush then took his arm and they walked inside along with the Cheneys. To honor the memories of those killed six years ago, all members of the White House staff were allowed to join in.

Less than an hour later, in a speech to family members of some of the Pentagon victims, Defense Secretary Robert Gates vowed that: "The enemies of America ... will never again rest easy, for we will hunt them down relentlessly and without reservation."

"Here at the Department of Defense, we pay an ongoing tribute with our firm commitment to defend the United States against any and all enemies, wherever they may exist," Gates said.

"And let there be no doubt that anyone wishing to revisit harm upon this country will find in the men and women of this department adversaries who have found clarity of purpose in their grief, a strength of resolve in their anger," he said.

Gates and Marine Gen. Peter Pace, chairman of the Joint Chiefs of Staff, laid a wreathe at the site of the attack, where a building stone charred by the fire that resulted has been engraved with the date and reset into the renovated structure.

Pace referred to the heated debate in the nation over the war in Iraq, saying there is a rightly a dialogue right now over how, where and when to try to defeat those who "on this day six years ago declared war on us."

Since then, 1.5 million service members have fought in Iraq and Afghanistan. He called Tuesday "a day of recommitment" by the military to continue defending the nation out of respect for those who lost on Sept. 11 and in the wars since.

"We cannot touch our loved ones today," Pace said, his voice choking with emotion. "Therefore we ask god to hug them for us, that they might know that we love them and we miss them and we'll serve this nation in their honor."