The early Monday morning fire, at 61 Academy St., started in the first-floor bathroom in the northeast section, said City of Poughkeepsie Fire Chief Mark Johnson.
The identities of the four found inside remain unknown, awaiting results from the Dutchess County Medical Examiner's Office.
The chief said at this point they (the investigating agencies) don't have any new evidence, and are waiting for forensics before a definitive cause can be determined.
At the time of the fire, all utilities had been turned off and there was no electrical service to the home.
The victims, who may have been homeless seeking warmth, might have used some sort of fire/fuel to keep warm or have light.
When firefighters arrived on the scene the entire first and second floors of the vacant home were engulfed in flames, the chief said.
"We pretty much knew right away that the home could not be saved," he said.
Once the fire had been extinguished around 4 a.m., with the help of Arlington and Fairview fire departments, firefighters discovered the first body around 4:15 a.m. on the second floor near a fire escape, Johnson said.
The second body was discovered about 20 minutes later on the third floor near a window, he added.
Two additional bodies were found on the first floor during an overhaul of the structure.
The ATF, City of Poughkeepsie Police Department, Dutchess County Sheriff's Office and Putnam Sheriff's Office are assisting with the investigation.
No firefighters were injured, the chief said.
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