Authorities initially said 26 people were killed in the fire, but one person previously thought to have died was later found with serious injuries, fire service spokesman Yuri Kislats said.
As of midday Friday no bodies had been gathered, and rescuers were hoping that others who had fled the fire would turn up later, Latvia's SWH radio reported, citing the rescue service.
Preliminary evidence suggested that the fire was caused by a short-circuit in the three-floor building's electrical wiring, Kislats said.
All the victims were residents of the convalescent home, rescue service spokeswoman Inese Veisa said.
She said the blaze caused the roof of the building to collapse. Eighty people were rescued but the search for survivors continued Friday as firefighters worked to put out the blaze.
Their efforts were hampered by extreme cold — the temperature reached minus 13 Fahrenheit — as several sources of water were frozen. Veisa said firefighters were forced to dig holes in the ice to find water.
Latvian Welfare Minister Dagnija Stake and Interior Minister Ivars Godmanis arrived at the scene to assess the situation.
"The tragedy is unspeakable," Stake told Latvian public radio, extending condolences to the victims' families.
The building was a manor built in 1890 and turned into a convalescent home in 1956. Agrita Groze, deputy state secretary of the Welfare Ministry, said it housed 97 patients, almost all of whom were disabled.
Kislats, however, said 88 patients were in the building at the time of the fire.
He said a preliminary investigation showed the fire may have been caused by a short-circuit triggered by overuse of electric appliances. The home's administration last year decided to use the third floor for accommodating patients without the necessary authorization from authorities, he said.
It was likely the third floor was not properly wired, and when patients began using additional heating equipment to stay warm, the electrical system overloaded, Kislats said.
However, Groze said the building, including the electrical wiring, passed an inspection last year and considerable repairs were made.