The Boston Fire Department reports "careless disposal of smoking material" started a three-alarm fire at 282 Hanover St. that killed two men - one after he jumped from the building to escape the flames.
The second man was found dead inside the building.
They are the first fire deaths in Boston this year, the department reports. Boston Police homicide detectives joined the investigation, standard for all fire deaths.
Firefighters responded at 1 a.m. for a fire on the third floor of the five-floor building. The smoker was in a second-floor bedroom, the department reports.
Two other adults and an infant were taken to to the hospital with injuries.
The fire caused an estimated $1.5 million in damage, the department says, adding 12 people were made homeless.