A large fire in Fairfax County destroyed an unfinished development of apartments and retail stores Saturday, shutting down traffic along Route 1 for several hours while firefighters battled the blaze and thick black smoke that could be seen for several miles.

County Fire Chief John S. Butler said there were no injuries from the five-alarm fire in the Alexandria section of Fairfax that started shortly before 10 a.m. inside the South Alex development on the 2800 block of Poag Street.

But before the fire was contained almost four hours later, it had spread to a row of townhouses also under construction and other surrounding buildings, Butler said.

“We are very concerned about spread,” Butler told reporters at the scene as firefighters from Fairfax and several other jurisdictions battled not just the flames but also gusty winds that fanned them.

“There are embers and hot material falling on buildings that are not yet on fire,” Butler said, adding that his department was worried that some tanks at the scene filled with propane would catch fire and explode.

While there were no reports of injuries, the fire stirred curiosity among onlookers, who posted dramatic video on social media of flames filling every window of one five-story building and captured the geyser of smoke that filled an otherwise clear sky.

The National Weather Service even weighed in, noting that smoke plume was showing up on radar and satellite feeds of the Washington region.

The damaged site is part of a wave of new development along Richmond Highway prompted by plans to widen the roadway and add rapid bus transit service in an area that has long been struggling.

With designs for a grocery store, a coffee shop and 400 condominium-style apartments, the site was expected to open in the summer of 2021, according to a description posted on its website. Sri Velamati, president of Combined Properties, the site’s developer, said in a statement that the company plans to rebuild.

“We are cooperating with officials as they try to determine the cause,” Velamati said.

Lori Windsor, a real estate agent responsible for a separate development of townhouses behind that property, said she learned about the fire shortly after closing the deal on one of the units Saturday.

Windsor got a call from the buyers who said they saw flames shooting up from a trash chute.

Within the hour, the townhouse she had just sold was enveloped by fire, along with 14 homes in that 41-home development, whose units sell for between $749,000 and just over $930,000, Windsor said.

A Fairfax fire spokeswoman said it could take several days before officials know how the fire started.