NEW YORK -- New York's new Sept. 11 museum is a monument to how the terror attacks that day shaped history, from its heart-wrenching artifacts to the underground space that houses them amid the remnants of the fallen twin towers' foundations. It also reflects the complexity of crafting a public understanding of the terrorist attacks and reconceiving ground zero.

The National September 11 Memorial Museum was set to be dedicated Thursday and open to the public May 21.

"It tells how in the aftermath of the attacks, our city, our nation and people across the world came together," former New York City Mayor Michael Bloomberg, the memorial foundation's chairman, said at a news conference Wednesday. "This museum, more than any history book, will keep that spirit of unity alive."

After Thursday's dedication, then six days of being open around-the-clock to Sept. 11 survivors, victims' relatives, first responders and lower Manhattan residents, the National September 11 Memorial & Museum before opening to the public.

Yet the memorial also reflects the complexity of crafting a public understanding of the terrorist attacks and reconceiving ground zero.

The museum faced financing squabbles and construction challenges. Conflicts over its content underlined the sensitivity of memorializing the dead while honouring survivors and rescuers, of balancing the intimate with the international.

The museum harbours both personal possessions and artifacts that became public symbols of survival and loss. There is the battered "survivors' staircase" that hundreds used to escape the burning skyscrapers, the memento-covered last column removed during the ground zero cleanup and the cross-shaped steel beams that became an emblem of remembrance. (An atheists' group has sued, so far unsuccessfully, seeking to stop the display of the cross).

Portraits and profiles describe the nearly 3,000 people killed by the Sept. 11 attacks and the 1993 trade centre bombing. Nearly 2,000 oral histories give voice to the memories of survivors, first responders, victims' relatives and others.

The museum also looks at the lead-up to Sept. 11 and its legacy.

Members of the museum's interfaith clergy advisory panel raised concerns that it plans to show a documentary film, about al-Qaida, that they said unfairly links Islam and terrorism. The museum has said the documentary is objective and its scholarship solid.

While some Sept. 11 victims' relatives have embraced the museum, others have denounced its $24 general-public ticket price as unseemly and its underground location as disrespectful, particularly because unidentified remains are being stored in a private repository there. Other victims' families see it as a fitting resting place.

The museum and the memorial plaza above it cost a total of $700 million to build. They will cost $60 million a year to run, more than Arlington National Cemetery and more than 15 times as much as the museum that memorializes the 1995 Oklahoma City bombing. Sept. 11 museum organizers have noted that security alone costs about $10 million a year.