After the new Millennium Tower Boston finally opened two years ago at the site of the old Filene’s Basement in Downtown Crossing, there was still one question that people had for Rosemarie Sansone, president and chief executive of the Downtown Boston Business Improvement District.

“They wanted to know: ‘What are they going to do about the old Barnes & Noble building?’” she recalls. “I didn’t have an answer for them.”

Now Sansone does. Ten months ago, L3 Capital LLC, a Chicago-based real estate investment firm that specializes in urban retail properties, purchased the six-story 399 Washington St. for $63 million. It had been more than a dozen years since Barnes & Noble closed its doors in the building in the center of Downtown Crossing.

In many respects, the 60,000-square-foot 399 Washington St. is a relatively non-descript building dating back to early in the last century. But it’s in a prime real estate location — just across the street from the old Filene’s Basement and now the gleaming new Millennium — and for years it was a beloved destination for people to browse for books or meet friends in the bustling retail district.

To many, the fact it sat empty for more than a decade has been puzzling: a reminder that for all that had changed in Downtown Crossing – including the influx of new retailers and residential buildings, younger high-tech office tenants and college dorms – there was something still missing.

That will change in early 2019, when L3 Capital plans to re-open the rebranded “DTX 399” with new retailers on the three lower floors and offices on the top three levels. No tenants have signed up yet, but L3 and Colliers International executives are optimistic their “vertical retail” plans will be a hit.

L3 hopes to announce a new tenant within 90 days, perhaps a “game-changer for Downtown Crossing,” said Peter Montesanto, senior vice president at Colliers International, which has been hired by L3 Capital as DTX 399’s broker.

For now, L3 Capital is focusing on the $15-million “back-to-its-studs” interior renovations, said Whitney Robinette, a director at L3 Capital, which has properties in Chicago, New York, Miami and Los Angeles.

Tentative renovation plans call for new electrical and elevator systems in the building, larger windows on the lower floors, and preservation of some historic building features at the top, Montesanto said.

`Location, Location and Location’

The renovation, rebranding and expected reopening of 399 Washington St. comes at a time when many brick-and-mortar retailers are sucking wind, especially retailers in the suburbs hit hard by e-commerce rivals and the changing shopping patterns of consumers.

But L3 Capital remains convinced that well-located downtown retail properties won’t be going out of fashion anytime soon, especially when so many people want to “live, work and play” in urban settings. 399 Washington, smack in the middle of a downtown retail and transit hub, is perfectly situated for urban consumers, Robinette said.

L3 Capital is no stranger to Boston, having owned and sold properties here in the past, and it’s been searching for a while for another investment in the city, Robinette said.

“L3 stands for location, location, location,” she said. “This (399 Washington property) was a great opportunity for us.”

Sansone, head of the Downtown Boston BID, said the re-opening has sparked a “huge celebration” among those who want to see a more vibrant 300 block of Washington Street.

As for the district in general, she said the area is doing quite well, thanks to the recent opening of major retailers like Primark and Old Navy and smaller shops like Caffe Nero. She also noted the huge popularity of the recently opened Roche Bros. supermarket in the Burnham Building.

Add to all of that new residential buildings and college dorms opened by nearby Suffolk University and Emerson College in the district, and the residential population of Downtown Crossing has risen from about 6,000 people a few decades ago to about 15,000 today, she noted.

Meanwhile, small high-tech companies – with younger workers – have been moving into Downtown Crossing in recent years, often into older buildings previously occupied by accounting and law firms.

“Ten or so years ago, you didn’t want to be near Downtown Crossing after 7 p.m.,” said Montesanto of Colliers. “Now it’s completely different.”

“Downtown Crossing has become a real 24-7 melting pot,” added Sansone.