Marking the 18th anniversary of the 9/11 attacks that irreversibly changed U.S. history, we take a look at some of the harrowing statistics from the terrorist attacks.

The Twin Towers were built in 1970

Total number killed in attacks (official figure as of 9/5/02): 2,819

Number of employees who died in Tower One: 1,402

Number of employees who died in Tower Two: 614

Ratio of men to women who died: 3:1

Age of the greatest number who died: between 35 and 39

Bodies found "intact:" 289

Body parts found: 19,858

Read more: Remembering the Irish Americans we lost in the 9/11 attacks

Number of people who lost a spouse or partner in the attacks: 1,609

Percentage of Americans who knew someone hurt or killed in the attacks: 20%

Number of people who died when American Airlines flight 11 from Boston, Massachusetts, to Los Angeles, California, crashed into the north tower of the World Trade Center: 92

Number of people who died when United Airlines flight 175  from Boston, Massachusetts, to Los Angeles, California, crashed into the south tower of the World Trade Center: 65

Number of people who died when United Airlines flight 93 from Newark, New Jersey, to San Francisco, California, crashed in rural southwest Pennsylvania: 45

Number of people who died when American Airlines flight 77,  from Washington to Los Angeles, crashed into the Pentagon: 64

Number of peole killed in the Pentagon: 125

Source: New York Magazine and The Guardian newspaper

Read more: 9/11 attack on World Trade Center seared my soul

*Originally published in 2011. 

Civilians flee the area as a tower of the World Trade Center collapses September 11, 2001, in New York City after two airplanes slammed into the twin towers in an alleged terrorist attack.Spencer Platt/Getty Images