Officials said eight families have been displaced and a firefighter was injured by falling debris while battling a large fire at the Midway Plaza on Flanders Road in the Niantic section of East Lyme.
Images and video from the scene show flames shooting through the roof of part of the building.
The call came in just before 5 p.m., police said.
There were people inside at the time of the fire but everyone was able to get out uninjured, according to police. About eight apartments and eight businesses were impacted.
"The second floor is pretty burnt throughout," said Flanders Fire Department Chief Chris Taylor. "The fire went through the roof and it collapsed on the right and side of the building. The first floor is all water damage."
East Lyme First Selectman Kevin Seery said officials have searched the building multiple times. Everyone living upstairs has been displaced as a result of the fire.
"We've reached out to resources to make sure they have a place to be located to tonight if they can't get into families and friends. We'll make sure they have a place to stay," Seery said.
A firefighter was transported to the hospital after getting hit by some falling debris, according to Taylor. He said he's going to be okay.
Flanders Road was closed between Industrial Park Road and Society Road but has since reopened.
One of the residents living on the second floor of the building said a firefighter saved her son.
"I drove all the way from Westbrook doing about 90 miles an hour and I came off the highway and I see a plume of smoke and I didn't know if he was okay," said Laurie Lennon of East Lyme.
Lennon said she's grateful to the young firefighter that got her son out.
"My son would have still been in the apartment if the firefighter didn't say 'it's clear - come,'" she said. "I want to find him and I want to say thank you."
Lennon said if there's one thing she learned, it's that getting renters insurance is important.
Crews were at the scene for several hours before clearing around 10 p.m.