Roughly two dozen people were forced to evacuate from an East Chatham apartment building early Monday after a fire police say was intentionally set in a vacant unit tore through the South Side complex, injuring one man.

       Firefighters were called to the 8200 block of South Drexel Avenue around 2:10 a.m. and soon elevated it to a second-alarm fire, sending additional crews to the scene. They located a 53-year-old man who was suffering from the effects of smoke inhalation and took him to Jackson Park Hospital for treatment, according to a media notification from Chicago police.

       Police said approximately 20 people evacuated the building, many of them from first- and second-story units. Larry Langford, the spokesman for the Chicago Fire Department, said most, if not all, of the residents who evacuated the building did so on their own before firefighters arrived, including the man who suffered minor injuries.

       Authorities also said the fire began in a vacant apartment on the horseshoe-shaped building’s third floor.

       “The fire in the vacant apartment was extinguished by CFD and the cause of the fire has yet to be determined,” the police statement said. A later update read: “The incident has been classified as arson.”

       In cases when a fire potentially becomes a crime, it’s the Police Department’s job to work with the Office of Fire Investigations, which takes the lead on determining the cause of a fire, although only police can determine if that cause was also criminal.

       “We do the science and they do the suspects,” Langford said. “We do not make a determination of arson, by regulation we do not declare arson. We can tell them a candle started the fire, they tell us who started the fire and why. We can narrow it down but we can’t do motive.”

       Asked what led to the determination the fire had been intentionally set, a police spokesman first directed questions to the Fire Department and when pressed said in an email: “According to the Chicago police report, Office of Fire Investigations related the fire to be arson.”

       The blaze was eventually extinguished around 3:30 a.m., according to a post from the Fire Department on Twitter.

       It was at least the third serious fire in Chicago in the past week, one of which resulted in the death this weekend of 81-year-old Venacio Delfino, a Northwest Side resident of a two-flat in the 5100 block of West Grand Avenue in Cragin that caught fire Saturday night.

       Another fire in the 3100 block of North Marmora Avenue in Belmont Cragin Thursday left a 37-year-old man dead and three others, including a Chicago firefighter, in critical condition.

       Check back for updates.

       kdouglas@chicagotribune.com

       Twitter @312BreakingNews

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