UPDATED 06/02/11 1:55 p.m.

CHICAGO (CBS) — People all around the world recognize the Willis Tower as a centerpiece of the Chicago skyline, and the tallest building in the Western Hemisphere.

But now, the iconic skyscraper known until two years ago as the Sears Tower is reportedly hundreds of millions of dollars in debt, and its owners are looking for help.

The Chicago Tribune reported the investment group that owns the building is about $780 million in debt, and the tower is now about 23 percent vacant.

Chicago-based American Landmark Properties Ltd., and two New York developers Joseph Moinian and Joseph Chetrit bought the tower in 2004 for about $900 million, the Tribune recalled.

They are now looking for a new investor that will help recapitalize the building. Spokesman Bill Utter tells CBS 2 that “a recapitalization effort has commenced with a select number of qualified investors.”

That investor would take partial ownership.

The building was renamed the Willis Tower in July 2009, after the London-based Willis Group Holdings insurance company leased 140,000 square feet in the building and moved in about 5900 employees, and also won naming rights.

Willis Group Holdings is not an owner of the building.

United Airlines, which is now owned by parent company United Continental Holdings following a merger with Continental Airlines, is also headquartered in the building. The airline is now moving its operations center from Elk Grove Village to the Willis Tower, the Tribune reported.

The 110-story skyscraper opened in 1973, as the headquarters for Sears, Roebuck & Company. Sears moved to Hoffman Estates in 1992, and the company lost its naming rights in 2003.

When it was completed, the former Sears Tower was the tallest building in the world at 1,451 feet. But it lost that claim in 1996 to the Petronas Towers in Kuala Lumpur, Malaysia. The Burj Khalifa in Dubai, United Arab Emirates is now the world’s tallest building, at 2.717 feet.

There has been no indication so far that the addition of a new investor would affect the naming rights for the Willis Tower in any way.