CHELMSFORD – An elderly married couple was killed and a 7-year-old boy was fighting for his life, scarred by third-degree burns in a three-alarm fire in Chelmsford last night that caved in more than a third of a 24-unit condominium complex.

“This has to be the worst in my career,” Chelmsford Deputy Fire Chief Gary Ryan, a 20-year veteran, told the Herald as smoke still billowed from the rubble, where furniture, kitchen cabinets and appliances dangled precariously over a flooded parking lot.

“The guys did a great job to rescue so many people, considering the circumstances,” Ryan said. “They saved a lot of lives in there.”

The boy, who neighbors said was visiting his grandparents from Maine, was airlifted to Massachusetts General Hospital in Boston by MedFight. Ryan said he is expected to survive.

The cause of the inferno, which officials said started shortly after 11 p.m. on the third floor, where the couple were found dead, is under investigation. Ryan did not have an immediate estimate on how many people got out because, as he explained, residents were streaming out all sides of Building 4 of Woodcrest Condominiums on Route 110.

Carol Cartier said the fire erupted just after she returned home last night from a ladies night out with five friends.

“Oh gosh, the flames,” she said, her eyes widening. “The flames were over the treetops. In fact, I thought all the trees were on fire.”

George Harrison, president of the Woodcrest Condominium Association, said the couple who died had lived there less than two years. The husband was in a wheelchair, having lost a leg to diabetes, he said.

“I don’t know what to say,” Harrison said with a blank, disbelieving stare. “I’m still in shock.”

Ryan said there were fire alarms sounding when first responders arrived.

“When we arrived on scene there were multiple rescues taking place. There were people on the front balconies, there was a rescue taking place on the back side, and we had a hose line deployed where the victims perished,” he said.

One rookie firefighter, still not fully trained, saved another elderly couple by coaxing them down an aerial ladder, he said.

“He starts the academy tomorrow,” Ryan said with amazement.

Of the wheelchair-bound man who perished, Ryan said, “They did their best to get him out. We were faced with deteriorating conditions so they had to get out. Things were collapsing.”

Chelmsford Deputy Fire Chief Michael Donoghue said local officials are working with the state Fire Marshal’s Office to determine the origin of the tragedy.

“We have accounted for everybody,” he said of the displaced residents.

Valeska Nova, 34, who lived on the first floor with six other family members including her husband, was on a ski trip in Killington, Vt., when she was called about the horrific news. Fortunately, the rest of her family was away.

Nova said she had left her diamond engagement ring at home, tucked in a drawer, and was thrilled that a firefighter was able to safely recover it for her.

“There are many elderly people who live in the building,” she said, still in her ski clothes. “We heard it was horrible. The fire started so fast. Now, two people are dead.”