Nearly 3,000 people were killed in the attacks on the World Trade Center, Pentagon and when a plane crashed in Shanksville, Pennsylvania.
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Remembrances were planned nationwide, including one by the Chicago Fire Department. Even as the rain fell, dozens of first responders were joined by city and state leaders for a moment of silence.
Illinois Gov. JB Pritzker and Chicago Mayor Lori Lightfoot were among the many in attendance.
CFD draped a flag on West Illinois Street to honor those who lost their lives that tragic day.
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Now, 21 years later, local officials said it's important to never forget the lives lost on 9/11.
"It was our first responders, police, fireman and even our flight crews on the airplanes," said 15th Ward Alderman Raymond Lopez. "As we've seen the narrative shift against our first responders, we must remember they are the true heroes amongst us."