This year marks the 18th anniversary of the terrorist attacks on September 11, 2001, which took the lives of nearly 3,000 people that morning and have taken many others since.

September 11 ultimately led to significant changes in counterterrorism measures in the United States and throughout the world. Within minutes of the attacks, Bureau officials activated the Strategic Incident Operations Center at FBI Headquarters in Washington, D.C., and by day’s end, the FBI had formed three command posts—one for each crash site.

In addition to supporting our partners in rescue and recovery efforts, the FBI also launched an investigation into the terrorist attacks. Codenamed PENTTBOM (short for Pennsylvania, Pentagon, and Twin Towers Bombing), the investigation was the largest ever conducted by the FBI, with more than 4,000 special agents and 3,000 professional staff employees helping in the recovery and subsequent investigation. Within hours, the FBI began to identify the 19 terrorists responsible for the attacks.

During the investigation, agents across the nation interviewed thousands of witnesses and sources and followed more than half-a-million leads worldwide, and the FBI collected and processed more than 150,000 pieces of evidence.

The September #ArtifactoftheMonth is a piece of steel from one of the eight buildings that comprised the World Trade Center. It potentially came from an elevator system or a subway rail. It represents just a small portion of the 180 million tons of debris recovered from Ground Zero after 9/11. The FBI received the steel from the Port Authority of New York and New Jersey in 2010.

Recovery workers cut various shapes and symbols—including crosses, shields, hearts, Stars of David, and even New York City skylines—from pieces of steel and gave them to victims’ families.

According to the National 9/11 Memorial and Museum, the pieces cut from the steel not only served as a physical reminder of a day that changed history, but also helped people impacted by the tragedy heal. Ironworker William Quinlan, one of the recovery workers who carved shapes from the steel, told the museum, “What I had done for the families, me and my crew, when we had a little slack time, we would cut out some crosses...and when they came down, we would give those crosses to the families. And they loved it; they really appreciated it. And it made us feel good.”

The FBI honors and remembers those who lost their lives during the attacks and those who have since passed away from health complications related to their rescue and recovery efforts.

For more information on the PENTTBOM investigation, please visit fbi.gov/history/famous-cases/911-investigation.