BOSTON--In the most deadly and horrific attack on the United States in its 225-year history, terrorists crashed two hijacked passenger jets into the World Trade Center towers in New York City, toppling the 110-story structures in a cloud of smoke and ash yesterday morning. Less than an hour later, another passenger jet crashed into the Pentagon in Washington, D.C. in an unprecedented attack upon the U.S. government.

221 passengers aboard the jets used in the attacks were killed, with an unknown number--most likely ranging in the thousands--killed in the three targeted buildings. A fourth hijacked jet crashed near Pittsburgh, Penn., killing an additional 45 people.

“Today we’ve had a national tragedy,” President George W. Bush said from Sarasota, Fla. yesterday morning as reports of the attacks began filtering in.

At 8:45 a.m. EST, American Airlines Flight 11, carrying 92 people from Boston’s Logan Airport, crashed into the north tower of the World Trade Center, igniting the top stories of the building in a ball of fire. Eighteen minutes later, United Airlines Flight 175, carrying 65 people bound for Los Angeles, crashed into the south tower of the center. The two crashes covered the New York City skyline in billowing smoke and flames, as hundreds of New Yorkers began streaming out of the twin steel and glass structures.

But before all of the nearly 50,000 World Trade Center employees could evacuate, the two towers collapsed to the ground in a scene similar to a staged building implosion, covering the downtown business district in ash and debris less than 90 minutes after the initial plane impacts.

Ten minutes before the south tower collapsed, an apparently coordinated strike occurred in the nation’s capital, as American Airlines Flight 77 crashed into a wall of the Pentagon, the headquarters for the U.S. Department of Defense.

And in Shanksville, Penn., United Airlines Flight 93 traveling from Newark to San Francisco crashed, in an apparently failed attempt to target Camp David, the presidential retreat in Maryland, located 85 miles southeast of the crash site.

The series of attacks sent the nation into a state of emergency: within minutes of the Pentagon crash, officials evacuated the White House and U.S. Capitol, and the Federal Aviation Administration (FAA) grounded all airplane takeoffs nationwide, the first such safety measure in U.S. history. All international flights were diverted to Canada, and U.S. borders were closed. There were no indications of when airports would reopen.