Over 1,000 victims of the of 9/11 attacks remain unidentified as the United States on Monday marked the 22nd anniversary of the attacks.

A commemoration ceremony was held on Monday at the National September 11 Memorial & Museum in Lower Manhattan, New York, where the 2,977 people killed in the September 11, 2001 attacks were honored.

Days ahead of the anniversary, the identification of two victims, a man and a woman whose names are withheld at the request of their families , from the deadliest terror attack on U.S. soil was announced.

The two new identifications represented the 1,648th and 1,649th persons identified since 2001 using advanced testing by New York City's DNA Laboratory, according to a press release by the mayor's office on Friday.

They were the first new identifications of World Trade Center victims since September 2021. However, 1,104 victims, 40 percent of those who died, remained unidentified, it said.

The number of 9/11 first responders who have died from Ground Zero-related health complications is nearly equal to the number of first responders who died during the attacks.

"When the towers fell on that terrible day, we lost 343 New York City Firefighters. ... In the years that have followed, over 341 more FDNY members have died from rare cancers and diseases caused by the toxic dust at Ground Zero," the Uniformed Firefighters Association of Greater New York wrote in a Facebook post on Monday.

As people commemorated those lost in the September 11 terrorist attacks in 2001, experts say the U.S. must reflect on the root causes of terrorism. 

The United States launched what it called the "Global War on Terrorism" following the attack in 2001. In the intervening years, at least 4.5 million people have died. Roughly a million from direct combat, and the remaining 3.5 million indirectly stemming from "destruction of economies, public services, and the environment," according to a report from the Brown University's Costs of War Project.

(With input from Xinhua)