Nearly 5,000 days after the attacks on 9/11 and seven years after construction on One World Trade Center began, tenants are moving into the tallest skyscraper in the Western Hemisphere.

The 16-acre site will always be holy ground. The plunging waterfalls in the footprints of the fallen towers will always be a pilgrimage site.

The 1,776-foot-tall tower, however, is not for memory; it is for work.

"We never stopped being there," said Ken Forsberg, Local 3 steward for One World Trade Center.

Local 3 members built the original World Trade Center 40 years ago and nearly 3,500 will tell their children and grandchildren they had a hand in the reconstruction. In 2003, Forsberg was one of the many journeyman who made the daily descent down the 480-foot ramp to rebuild what was lost. In 2009, he returned as steward for the whole site, a task he shared with others as the scope of the work expanded.

For some, it was too much.

"A few guys would tell me 'I can't come to the job. I get emotional. It's all I think about,'" Forsberg said. "I have nine years down there and an absolute attachment to the site. I still get choked up. The amount of time you spend doesn't toughen you any. It doesn't fade the raw memories."

Finishing it doesn't either. There is still the horror of the 2,753 people killed, including 21 IBEW members. Local 3's lost 17 are memorialized on the face of Local 3's hall. Four more were lost from Local 1212. On the back side of the memorial to the IBEW dead is a pledge to rebuild what was destroyed.

One World Trade Center is not the first, or even the second office building to open on the site. Building 7 opened in 2006 and Building 4 opened seven years later. The memorial opened on the 10th anniversary of the attacks and the long-delayed museum opened nearly two years ago.

But for Forsberg — for all of New York — without a flag flying over a finished Tower One, the pledge to rebuild was a promise unfulfilled.

Opening the doors of the tower to working men and women who will work in the building, not on it, is the start of a new page in the city's story. The hole in the skyline where the towers were hasn't been filled, but it is not as empty either.

"For years I'm looking around and watching this not be done. People came to the site and just saw fences up. It was like we were on our knees for a long time," said Elliot Hecht, Local 3 business representative for Lower Manhattan. "There were a lot of delays, but once construction started, we did what we do best: build. Now, you just look up now and think, 'This is really incredible.' We are very proud of what we have accomplished."

Ever since the days when members of Local 3 ran emergency services to keep the smoking wreck of Ground Zero lit, the IBEW has been there. At the peak of construction, Hecht said, 1,100 members passed through the layers of security each day.

There have been many milestones along the way to One World Trade Center's opening, markers celebrated with speeches and the tolling of bells from Trinity Church nearby. The May 2002 end of the recovery operation. The 30-ton Adirondack granite cornerstone, laid July 4, 2004. The raising of the first steel beam in April 2006 was broadcast live around the world. The foundation was completed January 2009. The steel plate that raised it higher than the Empire State Building was placed April 2012.

But for Hecht, the most emotional moment was May 10, 2013, when the 408-foot spire rose into place.

"When that spire went up and we knew the names of the lost Local 3 members were engraved on the beacon, you want to talk about feelings?" Hecht said. "That was a real special day."

The first tenants at One World Trade Center were from Condé Nast, publisher of GQ, Allure, Bon Appetit, Vanity Fair, Vogue and the New Yorker. Forsberg said he passed them in the halls, in the marbled lobby and on the street.

"There is me standing there watching them. We got butterflies. They're stepping out of a cab and they've got butterflies," Forsberg said. "It's not a construction site anymore. Now it's occupied by regular people doing everyday jobs."

They will occupy an unprecedented building, "the safest in the world" according to One World Trade Center's owner, the Port Authority of New York and New Jersey. The stairwells are 50 percent wider than required by code and pressurized to keep them smoke free. There is even a stairwell reserved for first responders. The stairs, sprinklers and elevators are protected by three feet of concrete. The first 185 feet of the tower is a blast-proof, above-ground bunker. All that safety takes up space, and although the top floor is called the 104th floor — same as the old towers — there are only 76 floors usable for office space.

Of course the work doesn't end for the IBEW now that tenants are moving in. Only 25 floors have been fit out, and unlike most other construction trades, electricians don't leave just because the windows are up. Forsberg expects a few hundred journeymen to be working on the tower for at least a few more years. Even when the final office space is finished, much work remains at the site. Looming largest, less than a block away, is the replacement for the destroyed commuter rail and subway station. The bleached backbone of its arching columns look more like an archeological dig than a future commuter hub.

But with every office that opens, or apartment that is filled, the glow of life gets brighter, the roots deeper, around that terrible scar.

"Lower Manhattan was a 9 to 5 community. You came for work and that was it. Now, this whole area is changing," Hecht said. "We've been building hotels, schools and new residential towers. It is back as part of the city."

It has become something different.

"They are moving in. We did it. Job well done," Hecht said.