The 9/11 museum at ground zero in New York City was once scheduled to open on Tuesday, the 11th anniversary of the Sept. 11, 2001, terrorist attacks. But today, while construction is 90 percent finished, the project has been bogged down for the last year by a squabble between New York City Mayor Michael Bloomberg and New York Gov. Andrew Cuomo over who will pay the $1 billion museum's operating costs, and who will oversee it. And while a handshake deal late Monday seemed to pave the way for progress, there's still no guarantee that both sides can truly overcome their differences and get the museum's doors open. Here's what you should know:

Why has the project been delayed?
On one side is the Port Authority of New York and New Jersey, jointly controlled by Gov. Cuomo and New Jersey Gov. Chris Christie. The Port Authority owns the World Trade Center site where the twin towers fell, and is responsible for building the museum, but not necessarily running it. On the other side is the 9/11 Memorial and Museum Foundation, chaired by Mayor Bloomberg, which actually controls the National September 11 Memorial and Museum. After the 10th anniversary of the attacks, the Port Authority essentially stopped construction, arguing that the foundation owed the Port Authority $150 million to $300 million. The foundation argued that if anything, the Port Authority owed them $100 million, for failing to complete the project on time. Another problem: A fierce argument over who would cover the expected $60 million in annual operating costs, and who would have oversight of the museum.

But there was a breakthrough, right?
Yes. With pressure mounting as the 11th anniversary approached, the two sides announced a deal late Monday that might get the work moving again. The Port Authority will drop its demand for repayment of the hundreds of millions it wanted from the foundation, in exchange for four seats on an eight-member oversight board. "My goal... has been to get construction on the museum restarted," said Bloomberg. "This agreement ensures that it will be restarted very soon and will not stop until the museum is completed."

When will the museum be done?
Time will tell. The foundation estimates that it will take a full year to build the cavernous 100,000-square-foot museum, which rests seven stories underground. It will take another three months or so to install the exhibits, which, for the time being, are gathering dust in Buffalo, N.Y., and Santa Fe, N.M., where they were assembled. It's conceivable that construction could wrap up by the 12th anniversary of the attacks, and that the museum could be completed —exhibits and all — by 2014. That would be five years after the initial target opening date. The plan now, though, is to try to open parts of the museum in phases, as construction allows, so it's conceivable that part of the museum could be open sooner.

What obstacles stand in the way?
Under the "memorandum of understanding" announced late Monday, the 9/11 foundation has agreed to always have enough money on hand to cover at least six months of operating expenses. That's a tall order. The $60 million it will take each year to operate the museum and memorial far surpasses the amount it takes to run other memorials. Gettysburg National Military Park costs $8.4 million, the USS Arizona Memorial at Pearl Harbor costs $3.6 million, and even Arlington National Cemetery is only $45 million. Running the 9/11 memorial site — which opened in 2011 and attracted 4.5 million visitors in its first year — is a uniquely challenging and expensive proposition.

What makes running the 9/11 memorial such a huge task?
Security, for one thing. The World Trade Center was twice attacked by terrorists — first in a 1993 bombing, and then in the 2001 suicide hijackings that brought down the twin towers. To keep visitors safe, the foundation plans to spend $12 million per year on private security, funneling visitors through airport-like security and patrolling the 16-acre site with armed guards.

Are there other issues beyond security?
Yes. Just operating the two massive fountains and waterfalls commemorating the towers' footprint costs $5 million annually. And the failure to get the museum open on time has caused fundraising to drop off, and also deprived the foundation of income from museum entry fees of $12 to $20 per person. With an expected 2 million annual visitors, the foundation was banking on those fees to cover the lion's share of its operating costs.

Will the museum be worth the wait?
The foundation promises it will be. The museum will be filled with 9/11 artifacts, including photos, videotapes, and voice messages victims left behind, as well as workplace memorabilia and recovered personal effects. There will even be a crushed firetruck — Ladder 3 — whose entire team of firefighters died rushing up to the fire in the north tower. "People walk up to the doors of the museum, and the doors are locked," says foundation board member Christine A. Ferer, whose husband, Port Authority executive director Neil D. Levin, died in the attack. "Locked inside is the full story of 9/11."

Sources: CNNGuardian, Huffington Post, New York Times, Washington Post