Filed at 10:11 a.m. ET

NEW YORK (AP) -- One World Trade Center tower has collapsed after two planes crashed into the upper floors of both World Trade Center towers minutes apart Tuesday in what the President Bush said was an apparent terrorist attack, blasting fiery, gaping holes in the 110-story buildings. There was no immediate word on deaths or injuries.

Shortly after, an explosion hit another building near the World Trade Center.

Within the hour, an aircraft crashed on a helicopter landing pad near the Pentagon, and the West Wing of the White House was evacuated amid threats of terrorism.

The president ordered a full-scale investigation to ``hunt down the folks who committed this act.''

One of the planes that crashed into the World Trade Center had been hijacked after takeoff from Boston, a U.S. official said, citing a transmission from the plane.

All planes were grounded across the country by the Federal Aviation Administration. All bridges and tunnels into Manhattan were closed down.

The twin disaster at the World Trade Center happened shortly before 9 a.m. and then right around 9 a.m.

Heavy black smoke billowed into the sky above the gaping holes in the side of the 110-story twin towers, one of New York City's most famous landmarks, and debris rained down upon the street, one of the city's busiest work areas. When the second plane hit, a fireball of flame and smoke erupted, leaving a huge hole in the glass and steel tower.

People ran down the stairs in panic and fled the building. Thousands of pieces of what appeared to be office paper came drifting over Brooklyn, about three miles away.

``Today we've had a national tragedy,'' Bush said in Sarasota, Fla. ``Two airplanes have crashed into the World Trade Center in an apparent terrorist attack on our country.'' He said he would be returning immediately to Washington.

Ira Furber, former National Transportation Safety Board spokesman, discounted the likelihood of accident.

``I don't think this is an accident,'' he said on CNN. ``You've got incredibly good visibility. No pilot is going to be relying on navigational equipment.''

``It's just not possible in the daytime,'' he added. ``A second occurrence is just beyond belief.''

Terrorist bombers struck the World Trade Center in February 1993, killing six people and injuring more than 1,000 others.

Several subway lines were immediately shut down Tuesday. Trading on Wall Street was suspended.

``The plane was coming in low and ... it looked like it hit at a slight angle,'' said Sean Murtagh, a CNN vice president, the network reported.

``I was watching TV and heard a sonic boom,'' Jeanne Yurman told CNN. ``The side of the World Trade Center exploded. Debris is falling like leaflets. I hear ambulances. The northern tower seems to be on fire.''

A senior government official, speaking on condition of anonymity, said the agency is pursuing reports that one or both of the planes were hijacked and that the crashes may have been the result of a suicide mission.

``It certainly doesn't look like an accident,'' said a second government official, also speaking on condition of anonymity.

In 1945, an Army Air Corps B-25, a twin-engine bomber, crashed into the 79th floor of the Empire State Building in dense fog.

In Florida, Bush was reading to children in a classroom at 9:05 a.m. when his chief of staff, Andrew Card, whispered into his ear. The president briefly turned somber before he resumed reading. He addressed the tragedy about a half-hour later.