Juan Estevez saw the horror from 25th Street, where he was working that day.

The planes had hit the World Trade Center towers, and Estevez, still unaware of the details behind the attack, watched stunned as flames and thick black sooty smoke enveloped the upper floors.

“The world became a different place on Sept. 11, our perception of the world [changed],” Estevez said in an interview with Fox News Latino at the site where the twin towers stood and new structures are being built.

Now, Estevez, who came to the United States from Spain when he was eight years old, is a lead project manager for the rebuilding of the World Trade Center. An enterprise, he says, that is more than just a major engineering project.

“It’s a message of hope, to make the world a better place, and be optimistic about the future” Estevez said.

The youngest of three brothers, Estevez, who speaks in an assured, but understated manner, is in his early 40s, and has a Master's degree in structural engineering from the University of Texas, and a Bachelor's degree in civil engineering from Virginia Tech.

With Tishman Construction since 2004, he has overseen the design and construction of many major projects in New York, including the 25-story Columbia University School and Faculty Housing and the Jewish Community Center of Manhattan.

Tishman, an AECOM company, also built the original World Trade Center.

The more than 1,000 workers currently toiling on the site, Estevez said, are keenly aware of what it symbolizes.

“As a New Yorker it’s a very dear project to everybody’s heart to rebuild the site,” he said. “Everybody in New York who lived here at the time had friends who worked at the towers.”

“I love seeing the new skyline of New York City, reminiscent of the skyline before Sept. 11,” he said. “It’s a great feeling, it fills a void in all of our hearts.”

Much work remains to be done on the inside of the building. Estevez said it is expected to be completed, and ready for occupancy, next year. Also under construction are Tower Three and Tower Four, as well as a transportation hub and vehicle security center.

Scott Rechler, the vice chairman of the Port Authority of New York and New Jersey, which owns the World Trade Center, said to reporters: "It's going to have a light that you can see from tens of miles away. And that light will change colors and in the next few months we are going to be activating that light, and it will be a beacon of hope just like the Statue of Liberty."

Rebuilding World Trade Center structures has posed formidable logistical challenges.

“The scale of the project is one of the great challenges,” Estevez said. “Building in Manhattan is very difficult, you don’t have a lot of storage space. You have to program all the activities for the day, all the deliveries for the day, almost to the minute. Building on top of a mass transit system, of the Path (train system), was a big challenge.”

But, Estevez said, “We’ve overcome them all."

One of the highlights of the rebuilding of the 104-floor structure was finishing the 20th floor, Estevez said.

“From a structural perspective, the structure is very complex from (floor) 20 down, it’s a lot of welding, a lot of man hours per piece, tons of steel. It’s a lot of work to get to the 20th floor.”

Once the 20th floor was complete, Estevez and officials of Tishman went to the deck and soaked up the moment.

From there, they could see the National September 11 Memorial nearby – with its two pools, one for each of the Twin Towers, and the engraved names of everyone who perished in the attacks.

“We have the memorial to reflect,” Estevez said, “and the tower as a sign of a better future.”

“I’m very proud to be associated with the most important project in the world.”

Elizabeth Llorente can be reached elizabeth.llorente@foxnewslatino.com

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