In what authorities believe was a coordinated terrorist attack, two planes struck the World Trade Center in New York City this morning, while another plane struck the Pentagon in Washington, D.C.

The first plane struck the trade center shortly before 9 a.m. Eastern. As camera crews filmed the billowing smoke from the WTC, a second plane – a commercial airliner – flew into the second WTC tower about 18-20 minutes later.

NBC reported that the second plane was a hijacked American Airlines Flight 11 from Boston to Los Angeles. Airline officials confirmed the loss of their Boston-to-L.A. flight.

Meanwhile, within an hour, a third plane struck a wing of the Pentagon opposite the Potomac River. Casualties are unknown at this hour, but officials believe the count may be very high.

Earlier, an anonymous caller claimed that the Democratic Front for the Liberation of Palestine was responsible for the attacks. About 10:00 a.m. Eastern, however, an official with that group denied it was responsible.

Also around 10:00 a.m., the Federal Aviation Administration issued an emergency order banning all U.S. flights from taking off from any airport in the U.S. Government officials that were out of town are trying to hail military flights to get back to offices in Washington. A half-hour later, the FAA ordered all international flights to land at airports in Canada.

Both the White House and the Pentagon have been evacuated, NBC reported, quoting anonymous government officials.

The NYC planes struck the 110-story trade center twin towers about three-quarters of the way to the top of the building. About an hour after the attacks, the top quarter of one of the towers collapsed into the streets below. Officials said there is a potential for the second building to collapse as well.

By late morning, smoke continued to billow out of the World Trade Center buildings, as emergency crews worked double time to evacuate casualties.

All financial markets have been shut down for the day, officials said. U.S. intelligence officials say they have no claims of responsibility in what analysts and experts say could be the worst terrorist attack in U.S. history.

President Bush has called the incidents a coordinated terrorist attack. He will be convening a meeting of national security advisors – analysts say away from the White House – to decide how to respond. Bush was in Florida to give a speech on education, but was flown back to Washington after receiving news of the attacks. Officials said Air Force One was escorted by U.S. fighter jets during the trip.

At about 10:16 a.m., Palestinian leader Yasser Arafat condemned the attacks in the U.S., and denied any Palestinian involvement.

By 10:30 a.m., the second tower collapsed into the streets. Witnesses say emergency workers were swarming around both buildings at the time of the collapse.

“This is a difficult moment for America,” Bush said in Florida, before departing to Washington. He vowed to “hunt down and find those folks” who committed the acts.

Witnesses said they saw U.S. Air Force fighters flying “cover” over Washington, D.C., shortly after rumors circulated around 10:45 a.m. that another jetliner may be inbound.

Analysts said the U.S. is vulnerable to terrorist attacks because of its “open society,” while others blamed a failure of U.S. intelligence.

Officials said that very few known terrorist groups had the power or ability to pull off the attacks, but they did not provide the names of those groups.

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