In pictures: Today marks the 18th anniversary of 9/11

The terrorist attack against the United States

The second tower of the World Trade Center bursts into flames after being hit by a hijacked airplane in New York September 11, 2001. PHOTO: Reuters

A helicopter flies past the World Trade Center after a commercial plane slammed into it in New York City, September 11, 2001. PHOTO: Reuters

The towers of the World Trade Center pour smoke shortly after being struck by hijacked commercial aircraft September 11, 2001. PHOTO: Reuters

A rescue helicopter surveys damage to the Pentagon as firefighters battle flames after a hijacked airplane crashed into the US military headquarters outside of Washington. PHOTO: Reuters

A group of firefighters walks amid rubble near the base of the destroyed South Tower of the World Trade Center in New York. PHOTO: Reuters

Rescue workers carry mortally injured New York City Fire Department chaplain, the Rev. Mychal Judge, from the wreckage of the World Trade Center in New York City September 11, 2001. PHOTO: Reuters

Cars smolder in the street as the destroyed World Trade Center burns in New York, September 11, 2001. PHOTO: Reuters

The wreck of the World Trade Center smolders in the background as a man passes a subway stop near the World Trade Center towers, September 11, 2001. PHOTO: Reuters

An American flag flies near the base of the destroyed World Trade Center, September 11, 2001. PHOTO: Reuters

The damaged area of the Pentagon building, where a commercial jetliner slammed into it September 11, is seen in the early morning at sunrise with the US Capitol Building in the background. PHOTO: Reuters

A lone police officer sits away from where family members of the victims of the September 11 attacks on the World Trade Center were gathering at the base of the "Ground Zero," September 11. PHOTO: Reuters