HARTFORD, Conn. (AP) — A fast-moving fire roared through a Connecticut home early Monday, killing two adults and two children and injuring five other people, including a firefighter, authorities said.

The fire in Hartford broke out around 2:30 a.m., and firefighters found heavy flames at the home upon their arrival, city Fire Department Chief Rodney Barco said. Crews were able to enter the home but were soon forced to retreat due to the intense heat and flames.

The names of the nine victims and further details about them were not immediately released. The five injured people — two adults, two children and a firefighter — were being treated at hospitals for undisclosed injuries that were not considered life-threatening. The firefighter was injured when he fell down a flight of stairs in “zero visibility” conditions caused by the smoke and other factors, Barco said.

The cause of the fire remains under investigation, Barco said.