Backgrounder: Attack on the U.S. -- What happened?

September 17, 2001 Posted: 1:29 PM EDT (1729 GMT)

(CNN) -- How do you explain the terrorist attack on America? CNNfyi offers this guide below and a corresponding lesson plan for students, parents and educators.

What happened?

On Tuesday morning, September 11, 2001, terrorists hijacked four commercial planes. Three of them slammed into buildings and a fourth crashed on land. It is the worst terrorist attack in the history of the United States.

In New York City:

A hijacked passenger jet out of Boston, Massachusetts, crashed into the north tower of the World Trade Center at 8:45 a.m. Tuesday, tearing a gaping hole in the building and setting it afire. About 18 minutes later, a second hijacked airliner, United Airlines Flight 175 from Boston, crashed into the south tower of the World Trade Center and exploded. Both buildings burned. Later in the morning the south tower and later the north tower collapsed, plummeting into the streets below. New York has reported 190 confirmed deaths and 4,957 people missing.

Click here for a map of the attacks.

Click here for information on the victims and the missing

In Washington D.C.:

A third hijacked airliner crashed into the Pentagon, sending up a huge plume of smoke. As a result of the crash, many government buildings including the White House were evacuated for fear they could be targets as well. A portion of the builder later collapsed. The Pentagon death toll, including the plane passengers, stands at 188 -- 186 missing and presumed dead, two confirmed dead.

In Pennsylvania:

A fourth hijacked airliner out of Newark, New Jersey, crashed in rural western Pennsylvania in the late morning. All of the plane's occupants, 45 people, died in the crash.

Who did this?

Investigations into who was behind Tuesday's terrorist attacks in the U.S. began just hours after the first plane struck the north tower of New York City's World Trade Center.

Within 48 hours some 4,000 special agents and 3,000 support personnel were assigned to the case, with about 400 FBI laboratory specialists deployed to examine the forensic evidence.

Almost immediately the finger of suspicion was pointed at exiled Saudi dissident Osama bin Laden and his group of followers known as al Qaeda, or "The Base."

Bin Laden has denied that he was involved in those attacks but nonetheless has said he supports the actions of those who carried them out.

The investigation is ongoing.

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