WASHINGTON — An aircraft crashed into the Pentagon Tuesday, and major government buildings from the Capitol to White House were evacuated after bomb threats.
President Bush said the two earlier plane crashes into the World Trade Center were "an apparent terrorist attack on our country."
The Federal Aviation Administration shut down all airplane traffic nationwide.
Major government installations, including Treasury and the State Department, were also evacuated.
A senior U.S. intelligence official, speaking on the condition of anonymity, said, "We don't know who's doing it. Clearly, it's terrorism related, a carefully coordinated attack. It's not the work of an unsophisticated enemy. It's too soon to say who."
Paul Begala, a Democratic consultant, said he witnessed an explosion near the Pentagon.
"It was a huge fireball, a huge, orange fireball," Begala said in an interview on his cell phone.
He said another witness told him a helicopter exploded. AP reporter Dave Winslow also saw the crash. He said, "I saw the tail of a large airliner. ... It plowed right into the Pentagon."
Gen. Richard Myers, vice chairman of the Joint Chiefs of Staff, said that prior to the crash into the Pentagon, military officials had been notified that another hijacked plane had been heading from the New York area to Washington. He said he assumed that hijacked plane was the one that hit the Pentagon, though he couldn't be sure.
Meanwhile, one of two planes that crashed into the World Trade Center was hijacked after takeoff from Boston, a U.S. official said, citing a transmission from the plane.
The second plane may have flown out of Newark, N.J., the official said, speaking on condition of anonymity.
Asked if there was any possibility the crashes were anything other than deliberate, the official said it appeared not to be an accident, the official said.
A somber Bush told an audience in Sarasota, Fla., gathered to hear an address on education that "this is a difficult moment for America." He said he would be returning immediately to Washington.
"Today we've had a national tragedy," Bush said. "Two airplanes have crashed into the World Trade Center in an apparent terrorist attack on our country."