(CNN) -- Here is some background on the terrorist attacks of September 11, 2001.
Facts: On September 11, 2001, 19 men hijack four fuel-loaded commercial airlines bound for west coast destinations. This terrorist attack on the United States is orchestrated by al Qaeda leader Osama bin Laden. A total of 2,977 people are killed in New York City, Washington, DC and outside of Shanksville, Pennsylvania, in the worst terrorist attack in U.S. history.
The Victims: At the World Trade Center site in Lower Manhattan, 2,753 people are killed when hijacked American Airlines Flight 11 and United Airlines Flight 175 are intentionally crashed in the north and south towers.
Of those who perish during the initial attacks and the subsequent collapses of the towers, 343 are New York City firefighters, another 23 are New York City police officers and 37 others are officers at the Port Authority.
The victims range in age from two to 85 years. Approximately 75-80 percent of the victims were men.
At the Pentagon in Washington, 184 people are killed when hijacked American Airlines Flight 77 crashes into the building.
Near Shanksville, Pennsylvania, 40 passengers and crew members aboard United Airlines Flight 93 die when the plane crashes into a field. It is believed that the hijackers crashed the plane in that location, rather than their unknown target, after the passengers and crew attempted to retake control of the flight deck.
Timeline: September 11, 2001 - 8:46am ET - American Airlines Flight 11 (traveling from Boston to Los Angeles) strikes the north tower of the World Trade Center in New York City. - 9:03am ET - United Airlines Flight 175 (traveling from Boston to Los Angeles) strikes the south tower of the World Trade Center in New York City. - 9:37am ET - American Airlines Flight 77 (traveling from Dulles, Virginia to Los Angeles) strikes the Pentagon Building in Washington. - 9:59am ET - South tower of WTC collapses in approximately 10 seconds. - 10:03am ET - United Airlines Flight 93 (traveling from Newark, New Jersey to San Francisco) crashes in a field near Shanksville, Pennsylvania. - 10:28am ET - North tower of WTC collapses. The time between the first attack and the collapse of both World Trade Center towers is 102 minutes.
December 13, 2001 - The U.S. government releases a tape in which Osama bin Laden takes responsibility for the attacks.
December 18, 2001 - Congress approves a measure to allow the president to designate September 11th as "Patriot Day" on each anniversary of the attacks.
January 23, 2004 - The New York City Medical Examiners Office has issued 2,749 death certificates, the first time since the attacks that the number of death certificates has matched the number of victims.
June 15, 2004 - The Victims Compensation Fund finishes its work processing death and injury claims from families and relatives of Sept. 11 victims. Families of those killed had until December 22, 2003 to apply for compensation. Families who agreed to get compensation from the federal fund agreed not to sue the airlines.
February 22, 2005 - Saying it has exhausted all DNA technology, the New York City Medical Examiner Office halts the process of identifying human remains from the September 11, 2001 terrorist attacks at the World Trade Center site. Of the 2,749 people known to have been killed at the World Trade Center site, only 1,585, or 58 percent, are identified by recovered physical remains.
April 2, 2007 - The remains of two more victims of 9/11 at the World Trade Center are positively identified. Of the 2,749 known deaths in the attacks in New York, 1,146 victims have yet to be linked to remains.
May 24, 2007 - The Chief Medical Examiner of New York City, Dr. Charles S. Hirsch, rules that the death of Felicia Dunn-Jones in 2002, from dust exposure, is directly linked to the 9/11 attack and therefore a homicide. This raises the 9/11 death toll to 2,750.
July 19, 2007 - The New York Medical Examiner's office announces that the remains of three more people are positively identified. 1,133 victims, 41 percent of the total, remain unidentified.
April 7, 2008 - The New York City Medical Examiner's Office announces that the remains of four more victims from the WTC have been identified. As of this date, 1,621 victims have been identified, leaving 1,129 unidentified.
January 2009 - The medical examiner's office rules that Leon Heyward, who died the previous year of lymphoma and lung disease, is a homicide victim because he was caught in the toxic dust cloud just after the towers collapsed. This brings the total number of 9/11 victim death toll to 2,752.
June 17, 2011 - The New York medical examiner rules that Jerry Borg's death on December 15, 2010 is a result of inhaling toxic substances from the dust cloud generated by the collapsing twin towers. This brings the number of victims at the World Trade Center site to 2,753 and the overall 9/11 victim death toll to 2,977.
August 23, 2011 - The New York City Medical Examiner's office announces that remains from the World Trade Center have been positively identified as 40-year-old Ernest James. As of this date, 1,632 victims have been identified, leaving as many as 1,121 unidentified.
February 10, 2012 - The New York City Chief Medical Examiner's office announces that remains from the World Trade Center have been positively identified as 42-year-old Karol Ann Keasler. As of this date, 1,633 victims have been identified, leaving as many as 1,120 unidentified.
July 5, 2013 - Remains are identified as firefighter Lt. Jeffrey P. Waltz, who was last seen in the north tower of the WTC.
Victim Compensation Fund: Operates from December 2001 to 2003.
The Victim Compensation Fund receives 7,408 applications for both death claims and personal injury claims.
The fund makes awards in 5,560 of those cases.
January 2, 2011 - President Obama signs James Zadroga 9/11 Health and Compensation Act of 2010, reopening and expanding the scope of the VCF.
Economic Impact: $500,000 - Estimated amount of money it cost to plan and execute the 9/11 attacks.
$123 billion - Estimated economic loss during the first 2-4 weeks after the World Trade Center towers collapsed in New York City, as well as decline in airline travel over next few years
$60 billion - Estimated cost of the WTC site damage, including damage to surrounding buildings, infrastructure and subway facilities.
$40 billion - Value of the emergency anti-terrorism package approved by the U.S. Congress on September 14, 2001.
$15 billion - Aid package passed by Congress to bail out the airlines.
$9.3 billion - Insurance claims arising from the 9/11 attacks.
It took 3.1 million hours of labor to clean up 1.8 million tons of debris.
The total cost of cleanup was $750 million.
Homeland Security: The Department of Homeland Security is created in response to September 11.
It merges 22 governmental agencies into one, including the Customs Service, the Immigration and Naturalization Service, the U.S. Coast Guard and the Federal Emergency Management Agency.
The Dept. of Homeland Security places 130 U.S. inspectors at ports in major European, Asian and Muslim nations, as well as strategically-located ports, to inspect cargo for nuclear, chemical or biological weapons being smuggled into the U.S.
The Homeland Security Advisory System is introduced on March 12, 2002.
The nationwide alert status has been raised from its current level of yellow (elevated) to orange (high) on five occasions. It has never been below yellow. New York City has remained at orange (high) since 9/11. In August 2006, the threat level for flights from the United Kingdom to the U.S. is raised to red, the first time that threat level has been used.
On April 26, 2011, the National Terrorism Advisory System (NTAS) replaces the color-coded Homeland Security Advisory System (HSAS). NTAS utilizes a vocabulary system including terms Elevated and Imminent.