The crash, part of a coordinated terrorist attack on the United States, took place less than an hour after two other hijacked commercial jets tore into the World Trade Center towers in New York City, with devastating results. A fourth jet, which also may have been targeted at Washington, crashed in southwestern Pennsylvania as it headed toward the capital.
After delivering a brief statement condemning the assault, Bush was flown to air force bases in Louisiana and Nebraska as a security precaution.
He arrived back at the White House last night and addressed the nation from the Oval Office.
"None of us will ever forget this day," Bush said, vowing to bring the terrorists and those who harbor them to justice for yesterday's "mass murder."
Authorities did not release a casualty count, but the attack on the Pentagon was the deadliest in the Washington area since the Civil War.
Airline officials said 64 passengers and crew were aboard when American Airlines Flight 77 crashed at high speed into the building's limestone façade about 9:40 a.m.
None aboard survived, said Defense Secretary Donald H. Rumsfeld. Among the victims was Barbara Olson, conservative Republican commentator and wife of U.S. Solicitor General Theodore Olson, who received two cell-phone calls from her as the hijacking was in progress.
Rumsfeld declined last night to characterize the number of lives lost inside the building except to say, "It will not be a few." The defense secretary, who wasn't injured, said he felt the shock of the crash as the jet hit between the first and second floors on the Pentagon's southwest side.
More than 23,000 people work at the Pentagon, the military's command-and-control center and the world's largest office building, most of which was undamaged. It is not known how many people worked in the area that was destroyed.
Rep. Ike Skelton, a Missouri Democrat, said he had been told that there were about 100 casualties at the Pentagon. He said he did not know how many of those had been killed and how many injured.
However, NBC News reported last night that there could be as many as 800 dead. A Pentagon source discounted that, saying the services believed that fewer than 500 personnel were missing and presumed dead.
At least 65 victims were taken to area hospitals by rescue workers, who struggled against the heat of fires that continued to burn into the evening. Hospital officials said the injuries ranged from minor to severe. At least 21 had been discharged by last night.
There were fears that the death count in Washington could exceed the 168 killed in the 1995 bombing of the federal building in Oklahoma City, until yesterday the deadliest act of terrorism in American history. Bush said last night the death toll from New York and Washington was in the thousands.
The plane that hit the Pentagon, a silver Boeing 757, was heavily laden with fuel for a coast-to-coast flight to Los Angeles. It sheared the tops off nearby light poles as it roared across a busy freeway and crashed.
With a crew of six and 58 passengers, the jet had lifted off from Dulles International Airport, about 25 miles away, about 8:21 a.m. It headed west before being commandeered over Ohio by knife-wielding terrorists and flown back toward Washington.
"This is indeed the most tragic hour in America's history," Sen. John W. Warner of Virginia, a senior Republican on the Armed Services Committee, told reporters at an news briefing in an undamaged part of the building.
Former Secretary of State George P. Shultz, on CNN, called it "an act of war against the Pentagon."