Sept. 11, 2001, was a dark day in American history that will forever be remembered.

The terrorist attacks took the lives of nearly 3,000 people. Commercial plane passengers, civilians on the ground and first responders all lost their lives that day in a horrendous act that is remembered with great despair each year. 

The events that unfolded on the morning of Sept. 11, 2001, will be forever remembered. 

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A timeline of the events was put forth by the 9/11 Memorial & Museum. Below are the moments that went on throughout the tragic day. 

The hijackers passed through security screening in Portland, Maine at 5:45 a.m.

There were 19 terrorists who passed through; they would later be split among four planes. 

Prior to 9/11, airports were not required to videotape security checkpoints and knives with less than a 4-inch blade were allowed on flights.

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Sept. 11, 2001, was a primary election day in the city. 

The polls opened up at 6 a.m. that morning, just slightly after the terrorists passed through security screening in Portland, Maine. 

The primary elections being held that day were for mayor, public advocate, comptroller and other city offices. 

The first of the four hijacked flights to take off was American Airlines Flight 11. 

This flight took off from Boston Logan International Airport in Massachusetts. 

There were 11 crew members and 76 passengers on board this plane with five hijackers.

The second flight to take off was United Airlines Flight 175.

This flight also took off from Boston, Massachusetts. 

There were nine crew members, 51 passengers and five hijackers on the plane.

Betty Ann Ong was a flight attendant on American Airlines Flight 11. She alerted American Airlines ground personnel of the hijacking by using an in-flight phone.

She made the call after a passenger was stabbed in first class. Ong's call lasted about 25 minutes. 

Shortly after she was on the phone with ground personnel, the hijackers turned off the plane's transponder, severing air traffic control's ability to monitor the plane's flight path.

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Shortly after, at 8:32 a.m., flight attendant Madeline Amy Sweeney called her friend, who was a manager at Boston Logan International Airport, and provided information about the hijacking, including a description of what the hijackers looked like.

American Airlines Flight 77 took off at 8:20 a.m.

This plane left from Washington Dulles International Airport. 

There were six crew members, 53 passengers and five hijackers on the aircraft. 

At this time, a message was transmitted that was only meant to go to those on board the plane, but the message was broadcast to air traffic control instead. 

The hijacker made another unintended transmission minutes later. 

At least one of the two unintended transmissions was noticed by the pilot of Flight 175 and was reported to the Federal Aviation Administration (FAA).

A large volume of workers started their day at the World Trade Center in New York City at 8:30 a.m.

Conferences began and events for later in the day were prepared.

One of the conferences of the day was the Risk Waters Group financial technology conference on the 106th floor of the north tower. At this time, 80 people had gotten together for the conference. 

Boston's air traffic control center contacted the military at 8:37 a.m. 

This came after hearing the transmission from the hijacker on Flight 11. 

The U.S. Air Force's Northeast Air Defense Sector (NEADS) prepared Air National Guard Jets to identify and follow the hijacked flight in response. 

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The last of the four hijacked planes to leave the airport was United Airlines Flight 93, which took off from Newark, New Jersey. 

This particular flight experienced a delay because of regular traffic. The flight was originally supposed to take off at the same time as the others.

United Airlines Flight 93 had seven crew members, 33 passengers and four hijackers aboard.

American Airlines Flight 11 slammed into the north tower of the World Trade Center in New York City. 

This was the first place to be hit by the terrorists. 

The 93rd to 99th floors were hit by the plane. Those on the plane and hundreds in the building were killed on impact.

Police, paramedics and firefighters were immediately sent to the scene. 

After the north tower was hit, President George W. Bush was alerted. 

He was told that a small plane hit the tower. 

At the time, he was visiting an elementary school in Sarasota, Florida.

The World Trade Center's south tower was declared secure at 8:55 a.m. 

"Your attention, please, ladies and gentlemen. Building Two is secure. There is no need to evacuate Building Two. If you are in the midst of an evacuation, you may use the reentry doors and the elevators to return to your office. Repeat, Building Two is secure," the announcement over the south tower's public address system said. 

Shortly after this announcement, an evacuation began.

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PAPD Sgt. Al DeVona issued an order to evacuate both of the towers at 8:59 a.m.

This call came just a few minutes after the tower was declared secure. 

The decision to evacuate was made a few short moments before the plane crashed into the south tower.

By about 9 a.m., people on board Flight 175 began making calls to family members. 

A flight attendant on the plane also managed to get in contact with the United Airlines operator in San Francisco to report the hijacking. 

That call came at 8:52 a.m. 

At 9:02 a.m., people started to evacuate the south tower. 

"May I have your attention, please. Repeating this message. The situation occurred in Building One. If the conditions warrant on your floor, you may wish to start an orderly evacuation," the announcement said. 

Just about a minute later, the tower was hit. 

The south tower was hit by United Airlines Flight 175 at 9:03 a.m. 

The plane crashed into floors 77 through 85. 

The number of people who were killed on impact is unknown. 

Many found themselves trapped inside. 

President Bush was alerted of the second plane strike. 

He left the elementary school in Florida at 9:35 a.m. and boarded Air Force One. 

The plane departed at 9:54 a.m.

At the same time, Mayor Rudolph Giuliani went to the NYPD command post.

Calls began to come through from those trapped on Flight 77. 

Flight attendant Renee A. May called her mother, Nancy May, and told her about the hijackers. 

After the call with her daughter, Nancy made a call to American Airlines.

Barbara K. Olson called her husband, U.S. Solicitor General Theodore Olson, who alerted federal officials.

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Another evacuation started on the morning of Sept. 11, 2001.

The Mayor's Office of Emergency Management was evacuated at 9:30 a.m. 

The headquarters were located at 7 World Trade Center. 

Evacuations continued during the morning of 9/11. 

Vice President Dick Cheney was moved to the Presidential Emergency Operations Center at 9:36 a.m. 

The Presidential Emergency Operations Center is located beneath the White House.

The bunker serves as a secure location for the president and others in case of an emergency situation. 

At 9:37 a.m., a third hijacked plane hit the terrorists' target. 

This was American Airlines Fight 77. 

The plane crashed into the Pentagon and killed 125 on the ground: 53 passengers on the plane and six crew members. 

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After the third plane crashed, the FAA grounded all flights in the United States. 

This stopped all air travel in the country for the time being.

At 9:45 a.m., following the three plane crashes, more government buildings started to be evacuated. 

The White House was evacuated at this time. 

The U.S. Capitol also began evacuations at 9:45 a.m. 

Around this time, phone calls were made from Flight 93. A total of 37 calls were made from this flight. 

One was a 911 call that was placed by passenger Edward Felt. 

He made the call from the bathroom of the plane. 

The south tower, which burned for 56 minutes after the initial crash, collapsed in just 10 seconds.

More than 800 civilians and first responders lost their lives. 

Also at this time, continuity of government was requested by the White House, according to the 9/11 Commission Report. 

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The continuity of government program "represents the President's intent that the United States have in place a comprehensive and effective program to ensure survival of our constitutional form of government and continuity of essential federal functions under all circumstances," according to the U.S. Department of Labor's website. 

Flight 93, which was initially heading for the U.S. Capitol, plunged into a field in Shanksville, Pennsylvania.

This occurred after passengers and crew made the decision to fight the hijackers.

The passengers and crew on the plane had learned about the prior attacks on the Twin Towers and Pentagon through phone calls. 

The Pentagon's E ring collapsed at 10:15 a.m. 

The collapse of the outer ring came shortly before the fall of the north tower in New York City. 

The north tower would collapse a little more than 10 minutes later. 

The north tower collapsed at 10:28 a.m.

The tower burned for 102 minutes before it came down. 

There were more than 1,600 killed. 

After the collapse of both towers, Giuliani ordered the evacuation of Lower Manhattan.

"I would like to take this opportunity to tell everyone to remain calm and, to the extent that they can, evacuate Lower Manhattan," he said. 

At this point, Giuliani sought shelter in a nearby office building.

VICTIMS TRAPPED IN TWIN TOWERS ON 9/11 SOUGHT HELP FOR THEMSELVES, OTHERS THROUGH PHONE CALLS

In the early afternoon, the last flight in the United States landed. 

This followed the FAA shutting down all U.S. flights. 

This marked U.S. airspace officially closing. 

At 12:30 p.m., a group of 14 survivors was found in the north tower wreckage. 

Thirteen of the people were first responders, with one civilian. 

They were found in stairwell B of the north tower.

Through the early afternoon, rescue efforts continued in the ruins.

This area became known as "Ground Zero." 

At 3 p.m. a survivor was found. 

The survivor found at 3 p.m. was Pasquale Buzzelli, who worked for the Port Authority. 

He was found in the north tower wreckage.

He was about 15 feet above the ground when he was found.

Later in the day, at 5:20 p.m., 7 World Trade Center collapsed. 

There were no causalities in the collapse. 

At this point, the building had already been evacuated.

On the night of 9/11, the president addressed the nation after the day's tragic events.

"The search is underway for those who are behind these evil acts. I’ve directed the full resources of our intelligence and law enforcement communities to find those responsible and to bring them to justice," President Bush said in his address. 

"We will make no distinction between the terrorists who committed these acts and those who harbor them," he continued. 

Later in the night, at 10:30 p.m., two more people were rescued: PAPD Officer William Jimeno and PAPD Sgt. John McLoughin. 

It took three hours to free Jimeno and eight to free McLoughin.

There was one more person rescued, Genelle Guzman, on Sept. 12. 

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