One World Trade Center, the unfinished skyscraper where the Twin Towers once stood, became the tallest building in New York City on Monday.

Still under construction more than a decade after the Sept. 11 attacks, the building's skeleton surpassed the Empire State Building's 1,250-foot-high observation deck, reaching 1,271 feet  with the addition of a few steel columns.

Once known as the Freedom Tower, One World Trade Center will eventually measure 1,776 feet, including a 408-foot-tall needle that will be attached to the roof.

"The latest progress at the World Trade Center is a testament to New Yorkers' strength and resolve -- and to our belief in a city that is always reaching upward," New York City Mayor Michael Bloomberg said in a statement.

"This building has been a labor of love for many, and I congratulate the men and women who have worked together to solve the challenges presented by this incredibly complex project," he added.