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.

Ground Zero in Boston

Since two of the four passenger jets hijacked yesterday took off from Boston’s Logan International Airport, the airport and surrounding hotels were transformed into an emergency command center as a reeling Massachusetts Port Authority responded to questions from panicked family members and both local and national media outlets.

The airport shut down at 9:06 a.m. and was eerily quiet an hour later. Stranded travelers searched for rental cars and hotel reservations as all airports across the country closed their gates.

The move left many passengers in disbelief.

Markas Muller, who was scheduled to fly home to Hanover, Germany yesterday, said he could not comprehend the images broadcast out of New York.

“I thought it was an advertisement for a movie or something,” he said slowly. “I didn’t think it could be real.”

Massport opened the Exchange Center on the Boston Fish Pier and outfitted the space with cots and food to help passengers like Muller, who were unable to find accomodations in the city’s overbooked hotels for the night.

About 50 would-be travelers spent the day at the makeshift center.

Massport also opened a family assistance center at the airport’s Hilton Hotel in conjunction with the airlines and American Red Cross. The center offered counseling to victims’ families.

At about 2 p.m. yesterday, police cars and Emergency Medical Services (EMS) trucks lined the entrance to the Hilton, while about a dozen state police officers guarded the doors.

“Our top priority right now is the families and the needs of these families,” said Massport Director of Public Safety Joseph Lawless. “We need to provide for the needs of the families and make sure we have a safe, secure airport.”

Nearly 50 reporters crowded into the lushly carpeted Grand Ballroom of the nearby Hyatt—turned into a media center—to await statements from Massport officials, trading notes and speculation as television reports played in the background.

At about 2:30 p.m., Lawless approached the podium, appearing worn and fatigued as he prepared for his brief address. He gave few specific answers, instead deferring to the ongoing FBI investigation.

Lawless said the airport had not been aware the passenger jets were hijacked before the crash was reported.

“Everything seemed normal when they left Logan,” he said.

When asked whether Logan Airport had suffered a breach of security, Lawless could not provide a definite response.

“I’m shocked—like everyone else is,” Lawless said. “We have a very high security standard here.”

America Responds

As a tense nation waited for answers, President Bush, who had been transported to Barksdale Air Force Base in Louisiana shortly after the attacks, condemned the terrorist actions as the work of “faceless cowards.”

“Make no mistake, the United States will hunt down and punish those responsible for these cowardly acts,” President Bush said. “The resolve of our great nation is being tested...we will show the world that we will pass this test.”

After a morning of chaos and confusion, speculation over who was responsible for the worst instances of terrorism ever committed on American soil quickly centered on Osama bin Laden, the Saudi millionaire exile who has been accused of masterminding past acts of terrorism against the U.S., including last year’s bombing of a U.S. navy ship near Yemen, as well as the 1998 bombings of two U.S. embassies in Africa. Islamic terrorists targeted the World Trade Center in 1993, killing six and injuring more than 1,000 others.

But while terrorists in the Middle East were the immediate focus of speculation, several groups—including bin Laden’s Al Quaida group and officials of the Taliban government in Afghanistan—denied taking any part in the attacks, saying they do not have the ability to mastermind such a large-scale operation.

Mullah Muhammad Omar, the Taliban’s supreme leader, issued a denial through a spokesperson, Abdul Hai Mutmain, who said, “Mullah Omar condemns this act. Mullah Omar says Osama is not responsible. We have brought peace to this country and we want peace in all countries.”

U.S. officials have said the full force of the U.S. military will be used to punish those responsible for yesterday’s attacks, as well as those who countries who have protected terrorists, such as Afghanistan. Early last night, explosions were reported in Kabul, Afghanistan, but the U.S. government has denied any role in those incidents.

After returning to Washington, President Bush addressed the country from the Oval Office at 8:30 p.m., providing a strong statement that America would seek retribution.

“The search is under way for those who are behind these evil acts,” Bush said. “I’ve directed the full resources for our intelligence and law enforcement communities to find those responsible and bring them to justice. We will make no distinction between the terrorists who committed these acts and those who harbor them.”

While the country’s attention will now turn to rescuing and caring for those who survived the New York and Washington attacks, as well as grieving for those who lost their lives yesterday, President Bush said that the U.S. would not let the attacks “frighten our nation into chaos and retreat.”

“This is a day when all Americans from every walk of life unite in our resolve for justice and peace. America has stood down enemies before, and we will do so this time,” Bush said. “None of us will ever forget this day. Yet we go forward to defend freedom and all that is good and just in our world.”

—Staff writers Lauren R. Dorgan and Zachary R. Heineman and the Associated Press contributed to the reporting of this story.

—Staff writer Imtiyaz H. Delawala can be reached at delawala@fas.harvard.edu. Staff writer Daniela J. Lamas can be reached at lamas@fas.harvard.edu.