Two firefighters suffered minor injuries Wednesday evening in a 4-alarm fire that caused an estimated $2 million in damage to a duplex in South Boston, a fire official said.

One firefighter was treated for heat exhaustion and another for a laceration, Deputy Fire Chief Jack Dempsey said at the scene.

The fire at 409 E. Seventh St. started in the basement on the left side of the duplex that was under construction and unoccupied, Dempsey said.

Six adults and one child who lived on the right side of the duplex got out safely, he said.

The fire, which was reported at about 5:20 p.m., spread quickly. Multiple alarms were sounded, drawing about 100 firefighters to the scene.


Because the building was under construction, the fire was able to move quickly from the basement through the roof, Dempsey said.

“There was no plaster on the walls . . . [so] there was nothing to contain it,” he said.

Dempsey praised firefighters for preventing the fire from spreading in the densely settled neighborhood.

“They did a great job keep it from spreading into the other buildings,” he said.

The site of the heavy flames, smoke and dozens of firefighters on the scene shocked some residents.

“All of the fire trucks were shooting water into the house and then we saw a bunch of flames pop through the roof which was pretty dramatic,” said Jack Bambach, 25, who lives two doors away from the duplex.

Celeste Wilson, 69, said she heard a big bang and then saw that the building was on fire.

“It really came up fast. It was like an explosion and you saw these huge flames,” said Wilson, who lives across the street “I was shocked at how high the flames were and I was scared because I saw people running out of the building next door.”


Peter Griglik said he was out walking his dogs around 5:20 p.m. when he heard a big pop.

“The flames burst out the top not 30 seconds after,” said Griglik, 55, who lives around the corner. “ It was quite remarkable.”

The cause of the fire is under investigation.

Stephanie Purifoy can be reached at stephanie.purifoy@globe.com. Follow her on Twitter @stephbpurifoy. Adam Sennott can be reached atadam.sennott@globe.com.