The two men were found unconscious on the top floor of the three-story house after they sent out mayday calls, Deputy Fire Chief Robert Petrucelli said.
Efforts to revive them using CPR were unsuccessful.
Lt. Steven Velasquez, 40, and 49-year-old Michel (Mitchell) Baik.were taken to hospitals and later pronounced dead.
The chief medical examiner's office says autopsies will likely be performed Monday.
"They were doing something that is quite routine, Fire Chief Brian Rooney said at a news conference Sunday. "Something obviously went wrong."
Firefighters battled temperatures that soared into the mid-90s. Several men experienced heat exhaustion, Deputy Fire Chief Robert Petrucelli said.
"In these temperature extremes, it's very punishing on the guys," he said.
Two other firefighters were injured, one with a twisted ankle and another who suffered smoke inhalation, Rooney said.
The deaths were the first for the Bridgeport Fire Department since 1999.
"We're the biggest city in the state but we're really just a small town and this is just devastating," Mayor Bill Finch said.
Velasquez, who had worked for the Bridgeport Fire Department since 1994, was promoted to lieutenant by Finch in February. He received the Medal of Merit, the department's third-highest award, in 2000.
Velasquez is survived by his wife and two children. Finch told reporters that Velasquez's wife held the youngest child when he delivered the news of her husband's death.
Baik had served since 2007 and is survived by his wife and three children. "Mitch was brand new," Finch said.
Firefighting was a second career for Baik, who previously worked in job development, the mayor said.
Velasquez and Baik were sent to the third floor of the building to rip down the ceiling, reach hot spots and search for anyone still trapped in the building, fire officials said.
The two men were found unconscious on the top floor of the three-story house after they sent out mayday calls, officials said.
Efforts to revive them using CPR were unsuccessful and they were declared dead at area hospitals.
The cause of the firefighters' deaths may have been smoke inhalation, Fire Captain Ed McCann told The Hartford Courant. The men were not burned, he said.
"They might have run out of air up there. We're not sure yet," McCann said.
The chief medical examiner's office told The Associated Press Sunday that autopsies will likely be performed Monday.
No residents of the house were injured, but three families were displaced.
Gov. M. Jodi Rell ordered flags at half-staff until the funerals, which have not been set.
The state fire marshal's office is investigating, Bridgeport officials said.
Walter Flyntz in 1999 was the last Bridgeport firefighter to die on the job.