Bells tolled at ground zero and solemn tributes unfolded around the U.S. as Americans looked back on Monday to the horror and legacy of the 2001 September 11 terrorist attacks. People gathered at memorials, firehouses, city halls, campuses and elsewhere to observe the 22nd anniversary of the deadliest terror attack on U.S. soil. 

On September 11, 2001, al-Qaida militants hijacked four airplanes and carried out suicide attacks against targets in the United States. Two of the planes were flown into the twin towers of the World Trade Center in New York City, a third plane hit the Pentagon and the fourth plane crashed in a field in Shanksville. Almost 3,000 people were killed in the terrorist attacks. 

U.S. President Joe Biden is due at a ceremony on a military base in Anchorage. First lady Jill Biden is due to lay a wreath at the 9/11 memorial at the Pentagon. In Pennsylvania, where one of the hijacked jets crashed after passengers tried to storm the cockpit, a remembrance and wreath-laying is scheduled at the Flight 93 National Memorial in Stoystown operated by the National Park Service.

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 20-year-long U.S. war in Afghanistan is a completely mistake and has brought huge disaster to local people," Afghan political analyst Sayed Qaribullah Sadat told China Media Group in a recent interview, noting that U.S.' two-decade military presence in Afghanistan, ending with a hasty withdraw, left an awful mess of unrest, division and broken families in the country.

"In the name of counter-terrorism and promoting peace and democracy, the United States brought about catastrophes to the Afghan people. Its actions never matched its words, as it is concerned about its own interests only. They do not abide by justice, peace, and true democracy. These words are merely tools for them to achieve their own goals," said the analyst.

"Our agriculture is not improved, nor did our economy get stronger. Our banks failed to develop. And our streets, we did not even have means to build a decent street. Many Afghans have been forced to leave their homes and flee to Iran, Pakistan, and other countries. This is very bad and negative impact," he said.

During the 20-year-long U.S. war in Afghanistan, the U.S. not only failed to achieve peace and development in Afghanistan, but instead resulted in a massive increase in civilian casualties and wrecked its economy. More than 174,000 people died directly in the war in Afghanistan, of whom over 47,000 were civilians, according to the Brown University's Costs of War Project.

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," the Brown University's Costs of War Project revealed.

(With input from AP)