President Bush, who will mark the remembrance of Sept. 11 by visiting three terrorist attack sites, plans to start the observances in prayer and close them with a prime-time address to the nation.

The White House on Tuesday released details of the president's Sept. 11 schedule, which will take him and first lady Laura Bush from a private morning church service in Washington, to a moment of silence observed at the White House at 8:46 a.m., EDT. That's the exact time that the first terrorist-hijacked jet slammed into the World Trade Center tower in New York. They will go from there to a ceremony at the Pentagon, which also was attacked on that fateful day.

The president and Mrs. Bush will then journey to Shanksville, Pa., and lay a wreath in the field where Flight 93 crashed, presumably en route to another target in Washington.

"Those who were here at the White House that day feel strongly about honoring those who gave their lives that day on Flight 93, particularly given the fact that most of us view that as savings the lives of those who were here at the White House that day," said White House deputy press secretary Scott McClellan.

That afternoon, at 4:30 p.m., Bush will lay a wreath at Ground Zero, site of the former Trade Center towers in New York. At 9:01 p.m., he is to address the nation from New York. White House press secretary Ari Fleischer promised a "respectful, solemn tribute to those who lost their lives on the attack on our country on Sept. 11 ... words of thanks and love to the families of those whose relatives were taken from us."

"And I think it will be a reminder of the importance of liberty, and how our United States stands strong throughout the world in promoting liberty," the spokesman added.

Meanwhile, Tom Ridge, director of Bush's Office of Homeland Security, said Tuesday he thinks U.S. security has improved significantly since the attacks.

"I think we've made substantial progress and I think we are substantially safer than we were on Sept. 11," he said on ABC's Good Morning America.

"We still have considerable work to do in the years and months ahead," Ridge acknowledged.

He said he knew of no "specific credible information" about any particular threat on Sept. 11.

"We have been in an elevated state of alert for quite some time," he said. "It's our job to worry about, be concerned about security every day," Ridge said on CBS' The Early Show.

"Obviously, this is a day that we are going to celebrate heroes. We are going to remember some extraordinary things that Americans did on behalf of one another. It's a day of mourning," he said.

He said he believed that "on that day, like every day since Sept. 11, America will be vigilant" and authorities at all levels of government "will continue to be on guard."