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Police, fire, and EMS first responders were joined by citizens nationwide in ceremonies marking the 18th anniversary of the 9/11 terrorist attacks that claimed the lives of nearly 3,000 people in New York, Virginia, and Pennsylvania.

The largest ceremony took place at the 9/11 Memorial plaza at the World Trade Center site in Lower Manhattan. The names of the men, women, and children killed as a result of the 9/11 attacks—as well as the 1993 World Trade Center bombing—were solemnly read aloud.

An honor guard—comprised of individuals representing the FDNY, NYPD, and Port Authority Police—also participated in the ceremony.

At the Pentagon—where 184 people were killed—President Donald Trump told survivors, family members, and first responders present that the U.S. continues to strike back in Afghanistan.

In Shanksville, Pennsylvania—where passengers aboard Flight 93 defeated the terrorists but died in the process as the airplane crashed in a field—Vice President Mike Pence said that the memory of those killed in the crash is "carved into the hearts and memories of the American people."

Police departments also posted messages of remembrance on social media.

The Duluth (MN) Police Department said on Facebook on Tuesday evening, "On this day...18 years ago 246 people went to sleep in preparation for their morning flights. 2,606 people went to sleep in preparation for work in the morning tomorrow. 343 firefighters went to sleep in preparation for their morning shift. 60 police officers went to sleep in preparation for morning patrol. 8 paramedics went to sleep in preparation for the morning shift. None of them saw past 10:00 a.m. Sept 11, 2001."

In California, The Redwood City Police Department posted, "The world as we knew it changed forever that morning in just 102 minutes. We mourn the 343 firefighters, the 60 police officers, the 8 EMTs and paramedics, and the thousands of civilians who lost their lives on September 11, 2001. May we never forget."

For the first time on this solemn anniversary, mourners were also able to visit a memorial to all those first responders who have died from 9/11-related illness.

Last spring, six granite slabs were installed at the World Trade Center Memorial honoring those officers and other first responders who have died from that illness.

A total of 23 officers with the New York Police Department died in the attack on the World Trade Center. Since that fateful day, 241 additional NYPD officers have died from 9/11-related illness. A total of 202 firefighters with the FDNY have died from the illness.

Related: How the Threat of Terrorism has Changed Since 9/11