By Hilary Russ

NEW YORK (Reuters) - The first office tower at Ground Zero since the September 11, 2001 attacks that destroyed the World Trade Center opened on Wednesday, marking a comeback for the Lower Manhattan site.

Sheathed in glass, 4 World Trade Center is the smallest of the four main towers on the site where 2,700 people died when hijacked airplanes crashed into the towers. It stands 977 feet tall, a shorter, simpler version of One World Trade Center, which will not be completed until early 2014.

"This isn't just a building. It's a symbol of our rebirth. It speaks to our promise to never forget," said Manhattan Borough President Scott Stringer at a ribbon-cutting ceremony attended by officials, friends, construction workers on the site and tourists snapping pictures.

Japanese architect Fumihiko Maki said he designed the building to have a "strong sculptural effect but with a quiet presence," and an "ambiance of calm and dignity."

The 72-story building stands empty at the moment, although two government agencies have signed leases for half of the building's space. Both the Port Authority of New York and New Jersey, which owns the site, and the city of New York committed to the space years ago to help jump start rebuilding efforts.

Since the attacks, disagreements among New York City, the state, the federal government, developers, insurers, victims' families and others have slowed construction on the 16-acre site. Developer Larry Silverstein, who held the lease to the site when it was attacked in 2001, has played a key role in shaping the project's design, security and cost.

"The world is recognizing that we've moved from 12 years of controversy and construction to having a real place that is coming back to life as a part of New York City," Janno Lieber, who oversees planning, design and rebuilding for Silverstein Properties, told Reuters.

Silverstein spokesman Dara McQuillan said the developer was not troubled that the 2.3 million-square-foot building is only half leased.

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In 2006, Silverstein opened 7 World Trade Center, just north of Ground Zero and the company was the only tenant in the building. But by 2011, it was fully leased with such tenants as Moody's Corp and Mansueto Ventures, which publishes Fast Company and Inc magazines.

"We learned at 7 World Trade Center that when you build state-of-the-art, green, high-tech office buildings, they lease quickly," McQuillan said.

Commercial real estate in downtown Manhattan rents for about $47 per square foot, about 35 percent less than rents in midtown Manhattan, according to commercial real estate services firm CBRE Group.

The skyscraper cost about $2 billion to build, including land lease costs, and was financed with $1.2 billion of tax-free Liberty Bonds and insurance proceeds.

On Monday, a council of urban designers decided that One World Trade Center, which is being built by Douglas Durst and the Port Authority, would be designated the tallest building in the United States.

They voted to count its spire in the total height of the building, which will reach 1,776 feet, a number chosen for the year the U.S. Declaration of Independence was signed. The spire on the original twin towers reached 1,727 feet and the Empire State Building's antenna spire reaches 1,454 feet.

A 9/11 museum is expected to open at the site in the next year. A transportation hub, designed by Spanish architect Santiago Calatrava, is scheduled to open in 2015.

The site will include two more office towers and some 550,000 square feet of retail space. Architect Frank Gehry, most famous for designing the contemporary Guggenheim Museum in Spain, has planned a performing arts center for the building.

(Reporting by Hilary Russ. Editing by Andre Grenon)