New York - In a vast underground hall beside twisted and graffiti-adorned steel remnants of New York's twin towers, President Barack Obama and other dignitaries joined rescue workers and families of Sept 11 victims on Thursday to dedicate the new National September 11 Memorial Museum.
Flags outside were at half-staff on the World Trade Center memorial plaza, where bronze panels bear the engraved names of the nearly 3,000 people who were killed in New York, Northern Virginia and Pennsylvania in 2001, and in the bombing of the World Trade Center in 1993.
About 700 guests attended the sombre ceremony held in the museum's Foundation Hall, 21m beneath ground level at bedrock.
Amazing Grace was sung by Rhonda LaChanze Sapp, an actress and Broadway singer who was pregnant when her husband, securities trader Calvin Gooding, was killed at the World Trade Center.
Foundation Hall is dominated by a giant slurry wall - the underground construction that survived the initial attacks and held back the waters of the Hudson River after the towers collapsed. In the hall's centre is the Last Column, the final steel beam to be removed from ground zero.
The museum, based where the twin towers once stood, near a permanent memorial that opened in 2011, appears modest with only one floor that can be seen from the outside compared with the towering skyscrapers that surround it.
The only work of art commissioned for the museum is Trying To Remember The Color Of The Sky On That September Morning, which looks like a decorative stone mosaic from a distance.
But as the viewer approaches the work by artist Spencer Finch, it becomes clear that the colour is simply watercolour paint on unframed paper, hung on a wire armature like the missing-person notices that papered the city after Sept 11.
The work is made up of 2,983 individual squares of paper - one square for every person killed in the Sept 11 attacks and in the 1993 bombing of the World Trade Center - each hand-painted a different shade of blue by Finch.
Visitors slowly march into the bowels of what once were the twin towers.
At the start of the exhibits, a map details the path of the four planes on a suicide mission that fateful day nearly 13 years ago.
The ramp then guides visitors down into the entrails of the ghost of the towers. The Last Column is seen, along with stairs from a nearby street that were used by hundreds of people to flee the site of the attacks. It takes time to reach the bottom of the museum, which houses the exhibits and the Foundation Hall.
The first exhibit, In Memoriam, pays tribute to the 2,977 people who lost their lives in the Sept 11 attacks, in addition to the six victims of a previous bombing of the World Trade Center in 1993.
The second exhibit, Historica, takes a more in-depth look at the events of Sept 11, including an analysis of the background to the attacks.
Visitors can hear the last telephone messages to loved ones left by New Yorkers trapped in the towers and the audio excerpts of the final moments of Flight 93 passengers, before hijackers crashed the airliner in a field in Shanksville, Pennsylvania. A total of 37 calls were made from the plane.
"Baby listen very carefully. We've been hijacked. Tell my children that I love them very much," air hostess CeeCee Ross-Lyles said in a call to her husband.
Mangled fire trucks offer silent testimony to the sacrifices of 343 firefighters who died in the deluge of fire and steel that accompanied the towers' collapse.
The visit ends in a room where a sevenminute video narrated by NBC Nightly news anchor Brian Williams explains "the rise of Al-Qaeda".
Museum president and former New York mayor Michael Bloomberg has hailed this as a museum that "more than any history book, will keep that spirit of unity alive". He justified the US$24 (S$30) entry fee by stressing that the museum has estimated operating costs of US$60 million a year.
New York Times, Agence France-Presse