WASHINGTON – A grim-faced President Bush mourned the deaths of thousands of Americans in Tuesday’s atrocities and vowed to avenge their killings. “Today, our nation saw evil,” he said.

In his first prime-time Oval Office address, Bush said the United States would retaliate against “those behind these evil acts,” and any country that harbors them.

Bush spoke from the Oval Office just hours after bouncing between Florida and air bases in Louisiana and Nebraska for security reasons. Fighter jets and decoy helicopters accompanied his evening flight to Washington and the White House.

With smoke still pouring out of rubble in Washington and New York, he said, “These acts shattered steel, but they cannot dent the steel of American resolve.”

Bush spoke for less than five minutes from the desk that Bill Clinton and John F. Kennedy used before him. Beside the door, a TelePromTer operator fed Bush the words that he and his speechwriters hastened to pen just an hour earlier.

He stumbled a couple of times even as he strove to maintain a commanding air. Aides pushed an American flag and one with the presidential seal behind him for the somber occasion.

Bush said the government offices deserted after the bombings Tuesday would open on Wednesday

He asked the nation to pray for the families of the victims and quoted the Book of Psalms, “And I pray they will be comforted by a power greater than any of us spoken through the ages in Pslam 23. Even though I walk through the valley of the shadow of death, I fear no evil for you are with me.”

The United States received no warning of the attacks on the Pentagon and New York’s World Trade Center towers, White House press secretary Ari Fleischer said.

U.S. officials privately said they suspected terrorism Osama bin Laden, protected by Afghan government, was behind the tragedies. The Afghan government has rejected the accusations.

“We will make no distinction between the terrorists who committed these acts and those who harbor them,” Bush said.

“Today, our fellow citizens, our way of life, our very freedom, came under attack in a series of deliberate and deadly terrorist acts.” He said thousands of lives were “suddenly ended by evil, despicable acts of terror,” Bush said.

The Oval Office address was his third statement on the tragedy.

He began his day in Sarasota, Fla., where he intended to talk about education. The remarks were scrapped and Bush headed to Louisiana.

He made a brief statement from a conference room at a Louisiana military base, assuring Americans that he was in regular contact with his command post in Washington: Vice President Dick Cheney, Defense Secretary Donald H. Rumsfeld and the White House national security team.

He then boarded Air Force One at 1:30 p.m. EDT for a secret destination that turned out to be Nebraska’s Offutt Air Force Base, home to the U.S. Strategic Command, which controls the nation’s nuclear weapons. Until three years ago, the Strategic Command also housed the so-called doomsday plane that had been specially equipped to serve as a flying White House in the event of nuclear war.

White House officials were sensitive to any appearance that Bush was not at the helm.

Fleischer said Bush wanted to be in Washington, where Cheney led the crisis operations center at the White House, but “he understands that at a time like this, caution must be taken” with his location.

At the first reports of attacks on New York’s World Trade Center, Bush told his Sarasota elementary school audience that he was hastening back to Washington. All of that immediately changed – and he was diverted to Louisiana – when a plane slammed into the Pentagon, and Washington, too, was under attack. On Capitol Hill, first lady Laura Bush, who was to have made her debut testifying before the Senate on education, tried to soothe a horrified nation.

“Parents need to reassure their children everywhere in our country that they’re safe,” she said, grim-faced, as she and Sen. Edward Kennedy, D-Mass., announced their hearing was postponed.

Mrs. Bush and a handful of aides were whisked by motorcade to a secret location away from the White House, which had been evacuated but for the small corps of foreign policy advisers who staffed the basement Situation Room.