A fire captain in his 50s was inside the vacant house in the 8700 block of South Burley Avenue, battling the fire, which started on back porches and spread inside, when he injured his knee. (WGN-TV)

A Chicago Fire Department captain injured his knee while battling a suspicious fire late Sunday at a vacant building on the Far South Side.

No one else was hurt during the blaze in a vacant house in the 8700 block of South Burley Avenue, said Chicago Fire Department Chief Lynda Turner.

The blaze was elevated to a still and box alarm at 9:45 p.m., bringing about 50 firefighters, five fire engines, three trucks, and a tower ladder to the scene of the 2½-story frame building, said firefighter Lorenzo Sanchez.

A fire captain in his 50s was inside the house battling the fire, which started on back porches and spread inside, when he injured his knee, said Sanchez.

The captain was taken in fair condition to Advocate Christ Medical Center in Oak Lawn, according to Sanchez.

Since no one was inside the building at the time, crews are considering the fire possibly suspicious, according to Sanchez.

The alarm was canceled at 10:30 p.m., but as of 11:30 p.m., crews were still pouring water on hot spots on the structure and "washing it down," Sanchez said.  

Chief Turner said the fire's cause is under investigation.

The Office of Fire Investigation was called to the scene. 

rsobol@tribune.com

Twitter: @RosemarySobol1