Today marks the 20th anniversary of the 911 terrorist attack. At 8:46 a.m. on Sept. 11, 2001, hijackers crashed American Airlines Flight 11 into the north tower of the World Trade Center in New York City. Six minutes later, the first contingent of New York City firefighters—two ladder and two engine companies—had arrived at the stricken building. They had just begun to climb a stairwell in an effort to reach people trapped on the upper floors, when another hijacked airliner, United Airlines Flight 175, at 9:03 a.m. struck the south tower. The 9/11 attacks not only became the single deadliest terrorist attack in human history, they were also the deadliest incident ever for firefighters, as well as for law enforcement officers in the United States. The New York City Fire Department lost 343 among their ranks, while 23 New York City police officers and 37 Port Authority officers lost their lives, according to the 9/11 Commission that investigated the attacks and emergency response. Learn more here. Source: history.com. Photo by Jose Jimenez/Primera Hora/Getty Images/Courtesy/history.com