The fire broke out at the Greenwood Health Center at about 2:30 a.m., forcing dazed elderly residents out on the street in below-freezing temperatures as firefighters tried to save those left behind. Many residents are bedridden or confined to wheelchairs. Others are blind or mentally retarded.
A nurse at the home told CBS News Radio the fire spread extremely fast.
The Hartford Fire Department sent multiple crews to the blaze and blocked off the area around Greenwood Street.
"Upon my arrival, it became apparent that all hands would be needed," said Hartford Fire Chief Charles Teale. "It's not just the fire that kills, it's the smoke."
Teale said 23 people were taken to hospitals, 10 of them in critical condition. Firefighters evacuated 100 people; in all, around 150 people live in the home.
At a news conference with Teale and other officials at City Hall, Mayor Eddie Perez offered his city's prayers and condolences to the victims and their families.
"The people of Hartford are with you and will help you overcome the tragedy," Perez said.
Arson investigators were on the scene, and authorities said a 23-year-old home resident was being questioned. Teale declined to comment on the possible cause of the fire, but said: "We do have substantial leads regarding that matter. We do have someone in custody at this time. There will be a lot of questions if her mental capacity was a factor."
Family members rushed to the fire to see if their loved ones were safe, and confused residents were seen seeking shelter.
One woman looking for her mother ran up the street sobbing, holding her temples and yelling in broken English.
"Where is the fire, where is the fire?" she cried.
The fire affected only one of the building's three sections. Family members looked through the windows of undamaged sections to try and find loved ones.
Donna Smith saw her father, Charles Harrison, through a window.
"Oh, Lord!" she screamed. "Thank you, Jesus." She bent down to kiss the snow.
Others were still searching. Jolie Marrero was looking for her uncle, 54-year-old Juan Sanchez. "He couldn't even have gotten out if wanted to. He's blind," she said.
Some families got information they did not want to hear. Luis Henriquez held a silver-framed school photo of his 17-year-old son, who had been in a coma for three years.
"One of the ministers told me he didn't make it," he said. "I want to know if the body's still here."
Temperatures were in the single digits, but none of the patients was in the cold for more than 10 minutes, Teale said. State Rep. Minnie Gonzalez, who was at the scene, said one woman pleaded, "Please take me to my room. I'm cold."
Three buses were brought to the scene to provide a warm shelter, and disaster crews from the American Red Cross and Salvation Army tried to give updates to family members.
The injured were taken to hospitals around the state, and many had respiratory injuries and burns, officials said.
The fire was extinguished a few hours after it started. Streets around the home were blocked off except for emergency vehicles.
Nine victims had been identified, but no names were immediately released, Hennessy said.
Fire Marshal William Abbott said the home was up to code, and extinguishers were present. There was no sprinkler system, but none was required. State officials said a Feb. 7 inspection found no fire or patient care violations.